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DSA Interview Questions and Answers, by Pattern

223 data structures and algorithms questions asked in coding interviews, grouped by the pattern that solves them. Each has the approach in a sentence or two, its time and space cost, and a link to a step-by-step visual solution.

Freshers: start with the Easy questions in each pattern before the Medium ones. Learning the pattern matters more than memorising any one answer, because interviewers change the details. For the order to study them in, see the 12-week FAANG preparation roadmap, and for a template per pattern, the patterns cheat sheet.

Two Pointers interview questions

How the two pointers pattern works

  1. Easy · Time O(n) · Space O(1)

    Reverse String

    Reverse an array of characters in place with two pointers that swap the ends and walk toward the middle.

    Answer: Solve Reverse String in place with two pointers: swap the first and last characters, move inward, and stop when they meet. O(n) time, O(1) space, 6 languages.

    Visual solution and code

  2. Medium · Time O(n) · Space O(1)

    Two Sum II - Input Array Is Sorted

    The integer array numbers is sorted in non-decreasing order and indexed from 1. Find the two entries whose sum equals target and return their positions.

    Answer: Solve Two Sum II in O(n) time and O(1) space: move two pointers inward on the sorted array and return 1-based indices.

    Visual solution and code

  3. Easy · Time O(n) · Space O(1)

    Valid Palindrome

    Check whether a string reads the same both ways, ignoring case and non-alphanumeric characters, with two pointers.

    Answer: Check a palindrome in O(1) space: walk two pointers inward, skip anything that is not a letter or digit, and compare ignoring case, step by step.

    Visual solution and code

  4. Easy · Time O(m + n) · Space O(1)

    Merge Sorted Array

    Merge two sorted arrays in place in O(m + n) by filling nums1 from the back with three pointers.

    Answer: Solve Merge Sorted Array in place: fill nums1 from the back with the larger of nums1[i] and nums2[j], so nothing is overwritten early. O(m + n), 6 languages.

    Visual solution and code

  5. Easy · Time O(n) · Space O(1)

    Remove Duplicates from Sorted Array

    Remove duplicates from a sorted array in place with a read pointer and a write pointer, and return how many unique values remain.

    Answer: Solve Remove Duplicates from Sorted Array in place: a write pointer k keeps the unique prefix while i reads ahead. O(n) time, O(1) space, code in 6 languages.

    Visual solution and code

  6. Easy · Time O(n) · Space O(1)

    Remove Element

    Remove every copy of a value from an array in place with a read pointer and a write pointer, and return how many values remain.

    Answer: Solve Remove Element in place: a write pointer k collects every value that is not val while i reads the array. O(n) time, O(1) space, with code in 6 languages.

    Visual solution and code

  7. Medium · Time O(n) · Space O(1)

    Rotate Array

    Rotate an array right by k steps in place with three reversals: the whole array, then the first k, then the rest.

    Answer: Solve Rotate Array in place with three reversals: reverse all, then the first k, then the rest. O(n) time, O(1) space, visualized step by step in 6 languages.

    Visual solution and code

  8. Easy · Time O(n) · Space O(n)

    Squares of a Sorted Array

    Return the sorted squares of a sorted array in O(n): the largest square is always at one end, so fill the output from the back with two pointers.

    Answer: Solve Squares of a Sorted Array in O(n) without sorting: compare the absolute values at both ends and fill the output from the largest square down. 6 languages.

    Visual solution and code

  9. Easy · Time O(n) · Space O(1)

    Is Subsequence

    Check whether s is a subsequence of t in one pass with two pointers, matching each character of s at its first chance in t.

    Answer: Solve Is Subsequence in O(n): walk t with one pointer and move a pointer in s on every match. Greedy, visualized, with code in 6 languages.

    Visual solution and code

  10. Easy · Time O(n) · Space O(1)

    Valid Palindrome II

    Decide whether a string becomes a palindrome after deleting at most one character, by trying both deletions only at the first mismatch.

    Answer: Solve Valid Palindrome II in O(n): move two pointers inward, and at the first mismatch check whether skipping the left or right character leaves a palindrome.

    Visual solution and code

  11. Medium · Time O(n²) · Space O(1)

    Longest Palindromic Substring

    Find the longest substring that reads the same both ways by expanding two pointers outward from every possible centre.

    Answer: Solve Longest Palindromic Substring by expanding around centres: 2n − 1 centres, two pointers stepping out while characters match.

    Visual solution and code

  12. Medium · Time O(n²) · Space O(1)

    Palindromic Substrings

    Count every palindromic substring of a string by expanding two pointers outward from each of its 2n − 1 centres.

    Answer: Count palindromic substrings in O(n²) time and O(1) space: expand two pointers from every character and every gap, counting each match.

    Visual solution and code

  13. Medium · Time O(n) · Space O(n)

    Encode and Decode Strings

    Encode a list of strings into one string and decode it back with length prefixes, read by two pointers.

    Answer: Encode and decode a list of strings with length prefixes: write length#word, then read it back with two pointers, safe even when words contain '#'.

    Visual solution and code

  14. Medium · Time O(n) · Space O(1)

    Container With Most Water

    Pick the two vertical lines that, with the x-axis as the floor, hold the most water between them.

    Answer: Find the container that holds the most water in O(n): start with the widest pair and move the shorter line inward. Compare it with checking every pair.

    Visual solution and code

  15. Medium · Time O(n²) · Space O(log n)

    3-Sum

    Find every group of three values, taken from different positions in the integer array nums, that adds up to 0, listing each distinct triplet once.

    Answer: Find every unique triplet that sums to zero: sort, fix one value, and close two pointers on the rest while skipping duplicates.

    Visual solution and code

  16. Medium · Time O(n²) · Space O(1)

    3Sum Closest

    Find the sum of three numbers closest to a target in O(n²): sort, fix one number, and move two pointers toward the target.

    Answer: Solve 3Sum Closest in O(n²): sort, fix one number, move two pointers toward the target, and keep the sum with the smallest distance. Visualized, 6 languages.

    Visual solution and code

  17. Medium · Time O(n³) · Space O(1)

    4Sum

    Find every unique quadruplet that sums to a target: sort, fix two numbers with nested loops, and sweep the rest with two pointers, skipping duplicates.

    Answer: Solve 4Sum in O(n³): sort, fix two numbers, sweep the rest with two pointers, and skip duplicates at every level. Watch the four pointers move. 6 languages.

    Visual solution and code

  18. Medium · Time O(n²) · Space O(log n)

    Valid Triangle Number

    Count how many triples of values from the integer array nums could be the three side lengths of a valid triangle.

    Answer: Count the triplets that form a triangle: sort, fix the longest side, and count every valid pair at once with two pointers.

    Visual solution and code

  19. Easy · Time O(n) · Space O(1)

    Move Zeroes

    Shift every 0 in the integer array nums to the back, in place, keeping the non-zero values in their original order.

    Answer: Move every zero to the end in place while keeping the order of the other values, with a read pointer and a write pointer.

    Visual solution and code

  20. Medium · Time O(n) · Space O(1)

    Sort Colors

    Sort an array of 0s, 1s and 2s (red, white and blue) in place and in one pass, so equal colours sit together in the order red, white, blue.

    Answer: Sort 0s, 1s and 2s in one pass and in place with low, mid and high pointers — the Dutch National Flag partition. Step through it.

    Visual solution and code

  21. Hard · Time O(n) · Space O(1)

    Trapping Rain Water

    Compute how much water can be trapped after raining.

    Answer: Compute trapped rain water in O(n) time and O(1) space by moving the pointer on the side with the lower wall. Step through it.

    Visual solution and code

  22. Medium · Time O(m · n) · Space O(m + n)

    Multiply Strings

    Multiply two numbers given as strings without converting them, using grade-school multiplication into an array of m + n digit positions.

    Answer: Solve Multiply Strings without BigInteger: digit i times digit j lands on positions i + j and i + j + 1. Step-by-step dry run and code in 6 languages.

    Visual solution and code

  23. Easy · Time O(max(m, n)) · Space O(max(m, n))

    Add Binary

    Add two binary strings with two pointers from the right and a carry: each column's sum mod 2 is the bit, sum / 2 the carry.

    Answer: Solve Add Binary by adding columns from the right: bit = total % 2, carry = total / 2, and keep going while a carry is left. Dry run and code in 6 languages.

    Visual solution and code

  24. Easy · Time O(max(m, n)) · Space O(max(m, n))

    Add Strings

    Add two non-negative numbers given as strings with two pointers from the right and a carry, without converting them to integers.

    Answer: Solve Add Strings without converting to integers: add digits from the right, write total % 10, carry total / 10. Dry run, edge cases and code in 6 languages.

    Visual solution and code

  25. Easy · Time O(n) · Space O(1)

    Plus One

    Add one to a number stored as an array of digits: walk from the end, turn trailing 9s into 0s, and grow the array only when every digit was 9.

    Answer: Solve Plus One by walking from the last digit: a digit under 9 takes the +1 and you return, a 9 becomes 0 and carries. All 9s grow the array. 6 languages.

    Visual solution and code

Hash Map interview questions

How the hash map pattern works

  1. Easy · Time O(n) · Space O(n)

    Two Sum

    Find the indices of two numbers that add up to a target in one pass with a hash map.

    Answer: Solve Two Sum in one pass: store each number, look up its complement, and watch the map fill step by step. Brute force and.

    Visual solution and code

  2. Easy · Time O(n) · Space O(n)

    Contains Duplicate

    Return true if any value appears at least twice in the array, using one pass with a hash set.

    Answer: Solve Contains Duplicate in one pass: check each number against a hash set, stop at the first repeat, and watch the set fill step by step.

    Visual solution and code

  3. Easy · Time O(n) · Space O(k)

    Valid Anagram

    Decide whether two strings are anagrams by counting letters in a hash map.

    Answer: Check whether two strings are anagrams with one hash map: count every letter of s, spend them on t, and watch the counts rise and fall step by step.

    Visual solution and code

  4. Medium · Time O(n · k log k) · Space O(n · k)

    Group Anagrams

    Group words that are anagrams of each other, using each word's sorted letters as a hash map key.

    Answer: Group anagrams in one pass: sort each word's letters into a key, collect words under their key in a hash map, and watch every group form step by step.

    Visual solution and code

  5. Medium · Time O(n) · Space O(n)

    Top K Frequent Elements

    Return the k most frequent values in an array in O(n) with a count map and buckets indexed by frequency.

    Answer: Find the k most frequent elements in O(n): count with a hash map, drop each value into a bucket by its count, and read the buckets from the top down.

    Visual solution and code

  6. Medium · Time O(n) · Space O(n)

    Longest Consecutive Sequence

    Find the length of the longest run of consecutive integers in an unsorted array in O(n) with a hash set.

    Answer: Find the longest consecutive run in O(n) without sorting: put the numbers in a hash set and count each run only from its start, step by step.

    Visual solution and code

  7. Medium · Time O(81) · Space O(81)

    Valid Sudoku

    Check that a partly filled Sudoku board has no repeated digit in any row, column or 3×3 box, in one pass with hash sets.

    Answer: Solve Valid Sudoku in one pass: a set for every row, column and 3×3 box, with box = (r / 3) * 3 + c / 3. Watch each digit get checked.

    Visual solution and code

  8. Easy · Time O(n) · Space O(1)

    Majority Element

    Find the value that appears more than n/2 times: count with a hash map, or use Boyer-Moore voting for O(n) time and O(1) space.

    Answer: Solve Majority Element with a hash map count, then in O(1) space with Boyer-Moore voting: a candidate and a count that cancels. Code in 6 languages.

    Visual solution and code

  9. Easy · Time O(n) · Space O(k)

    Isomorphic Strings

    Check whether two strings are isomorphic with two hash maps, s to t and t to s, that must stay consistent for every pair of letters.

    Answer: Solve Isomorphic Strings in O(n) with two hash maps, s→t and t→s; see why one map misses cases like badc and baba. Visual walkthrough, code in 6 languages.

    Visual solution and code

  10. Easy · Time O(n + m) · Space O(n + m)

    Word Pattern

    Check whether a string of words follows a letter pattern with two hash maps, letter to word and word to letter, after checking the counts match.

    Answer: Solve Word Pattern in O(n + m): split s, check the word count, then keep letter→word and word→letter maps consistent. Visual walkthrough, code in 6 languages.

    Visual solution and code

  11. Easy · Time O(m + n) · Space O(1)

    Ransom Note

    Decide whether a ransom note can be built from a magazine's letters by counting the magazine in a hash map and spending one count per note letter.

    Answer: Solve Ransom Note in O(m + n): count the magazine's letters, then spend one per note letter; fail when a count hits 0. Visual walkthrough, 6 languages.

    Visual solution and code

  12. Easy · Time O(n) · Space O(1)

    First Unique Character in a String

    Find the index of the first non-repeating character with two passes: count every letter in a hash map, then return the first index with count 1.

    Answer: Solve First Unique Character in a String in O(n): count letters in a hash map, then scan again for the first count of 1. Visual walkthrough, 6 languages.

    Visual solution and code

  13. Easy · Time O(n) · Space O(1)

    Roman to Integer

    Convert a Roman numeral to an integer in one pass with a symbol-value map: subtract a symbol that is smaller than the next one, otherwise add it.

    Answer: Solve Roman to Integer in one pass: map each symbol to its value, subtract it when the next symbol is bigger, add it otherwise. Visual walkthrough, 6 languages.

    Visual solution and code

Sliding Window interview questions

How the sliding window pattern works

  1. Medium · Time O(n) · Space O(min(n, charset))

    Longest Substring Without Repeating Characters

    Find the length of the longest substring without repeating characters.

    Answer: Solve Longest Substring Without Repeating Characters in O(n): store each character’s last index and jump the window past repeats.

    Visual solution and code

  2. Medium · Time O(n) · Space O(1)

    Longest Repeating Character Replacement

    Replace characters to get the longest repeating character substring.

    Answer: Find the longest substring you can turn into one letter with at most k replacements: a sliding window with letter counts.

    Visual solution and code

  3. Hard · Time O(|s| + |t|) · Space O(k)

    Minimum Window Substring

    Find the shortest part of s that contains every character of t, duplicates included, with a window that grows until it covers t and shrinks while it still does.

    Answer: Solve Minimum Window Substring in O(|s| + |t|): extend right until the window covers t, then shrink from the left while it still does.

    Visual solution and code

  4. Medium · Time O(n) · Space O(1)

    Minimum Size Subarray Sum

    Find the shortest subarray whose sum reaches a target in O(n) with a sliding window that grows on the right and shrinks from the left.

    Answer: Solve Minimum Size Subarray Sum in O(n): grow the window right, and while its sum reaches the target, record its length and shrink from the left. 6 languages.

    Visual solution and code

  5. Medium · Time O(n) · Space O(1)

    Max Consecutive Ones III

    Find the longest run of 1s after flipping at most k zeros, as the longest sliding window containing at most k zeros.

    Answer: Solve Max Consecutive Ones III in O(n): keep a window with at most k zeros, shrink it from the left when one zero too many arrives, track the longest.

    Visual solution and code

  6. Medium · Time O(n) · Space O(1)

    Fruit Into Baskets

    Pick the most fruit with two baskets from consecutive trees: the longest sliding window with at most two distinct values.

    Answer: Solve Fruit Into Baskets in O(n): it is the longest window with at most two distinct values. Count kinds in a map and shrink when a third arrives. 6 languages.

    Visual solution and code

  7. Easy · Time O(n) · Space O(min(n, k))

    Contains Duplicate II

    Check whether two equal values sit at most k positions apart, with a sliding set that holds only the last k values.

    Answer: Solve Contains Duplicate II in O(n): keep a set of the last k values; if the next value is already in it, two copies are at most k apart. Code in 6 languages.

    Visual solution and code

  8. Medium · Time O(k) · Space O(1)

    Maximum Points You Can Obtain from Cards

    Find the maximum score you can get by taking k cards from either end of the row.

    Answer: Take k cards from either end for the highest score: start with all k from the left, then trade one left card for one right card at a time.

    Visual solution and code

  9. Medium · Time O(n) · Space O(k)

    Maximum Sum of Distinct Subarrays With Length K

    Find the maximum sum of a fixed-size subarray where all elements are distinct.

    Answer: Find the largest sum of k consecutive distinct values: slide a window with a running sum and a count map, and skip windows that repeat a value.

    Visual solution and code

  10. Medium · Time O(n) · Space O(1)

    Permutation in String

    Check whether s2 contains a permutation of s1 by sliding a fixed window of len(s1) and comparing letter counts.

    Answer: Solve Permutation in String with a fixed sliding window: keep 26 letter counts for the last len(s1) characters of s2 and compare them with s1's. 6 languages.

    Visual solution and code

  11. Medium · Time O(n) · Space O(1)

    Find All Anagrams in a String

    Find every start index of an anagram of p in s with a fixed-size sliding window that keeps letter counts up to date as it moves.

    Answer: Solve Find All Anagrams in a String in O(n) with a fixed sliding window: add the entering letter, drop the leaving one, compare 26 counts. Code in 6 languages.

    Visual solution and code

  12. Hard · Time O(n) · Space O(k)

    Sliding Window Maximum

    Return the maximum of every window of size k in O(n) with a monotonic deque that keeps only indexes that could still become a maximum.

    Answer: Solve Sliding Window Maximum in O(n) with a monotonic deque: drop expired indexes from the front, smaller values from the back.

    Visual solution and code

Intervals interview questions

How the intervals pattern works

  1. Medium · Time O(n log n) · Space O(n)

    Merge Intervals

    Merge all overlapping intervals.

    Answer: Solve Merge Intervals in O(n log n): sort by start, then extend the last merged interval or append a new one. Step through it.

    Visual solution and code

  2. Medium · Time O(n) · Space O(n)

    Insert Interval

    Insert a new interval into a sorted non-overlapping list.

    Answer: Insert a new interval into a sorted, non-overlapping list in O(n): copy what ends before it, absorb what overlaps it, then copy the rest.

    Visual solution and code

  3. Medium · Time O(n log n) · Space O(log n)

    Non-overlapping Intervals

    Find the minimum number of intervals to remove to eliminate all overlaps.

    Answer: Find the fewest intervals to remove so none overlap: sort by end, keep each interval that starts at or after the last kept end, and count the rest.

    Visual solution and code

  4. Easy · Time O(n log n) · Space O(log n)

    Meeting Rooms

    Determine if a person can attend all meetings.

    Answer: Decide whether one person can attend every meeting: sort by start time and check each meeting against the one before it.

    Visual solution and code

  5. Medium · Time O(n log n) · Space O(n)

    Meeting Rooms II

    Find the minimum number of conference rooms required.

    Answer: Find the minimum number of meeting rooms in O(n log n): sort start and end times separately and walk them with two pointers.

    Visual solution and code

  6. Medium · Time O(n log n) · Space O(log n)

    Min Arrows to Burst Balloons

    Find the minimum number of arrows to burst overlapping balloons.

    Answer: Find the fewest arrows that burst every balloon: sort balloons by end, shoot at each uncovered balloon’s end, and count the arrows.

    Visual solution and code

Linked List interview questions

How the linked list pattern works

  1. Easy · Time O(n) · Space O(1)

    Reverse Linked List

    Reverse a singly linked list in place by flipping one link at a time.

    Answer: Reverse a linked list in place with three pointers and watch each next link flip one node at a time. Iterative and recursive.

    Visual solution and code

  2. Easy · Time O(n) · Space O(1)

    Middle of the Linked List

    Find the middle node of a linked list in one pass with slow and fast pointers: when fast reaches the end, slow is halfway.

    Answer: Solve Middle of the Linked List in one pass: slow moves 1, fast moves 2, and slow is the middle when fast hits the end. Visual walkthrough, code in 6 languages.

    Visual solution and code

  3. Easy · Time O(m + n) · Space O(1)

    Intersection of Two Linked Lists

    Find the node where two linked lists merge in O(1) space: two pointers that each walk one list then the other meet at the intersection or at NULL.

    Answer: Solve Intersection of Two Linked Lists in O(m + n) time, O(1) space: each pointer walks its list then the other, so both meet at the shared node. 6 languages.

    Visual solution and code

  4. Easy · Time O(n + m) · Space O(1)

    Merge Two Sorted Lists

    Merge two sorted linked lists into one sorted list by splicing nodes after a dummy head.

    Answer: Merge two sorted linked lists in place: hang nodes off a dummy head, take the smaller head each step, and attach the leftover list at the end.

    Visual solution and code

  5. Medium · Time O(max(m, n)) · Space O(max(m, n))

    Add Two Numbers

    Add two numbers stored as reversed linked lists of digits, column by column with a carry, building the result through a dummy head.

    Answer: Solve Add Two Numbers: walk both reversed digit lists, add each column and the carry, append the digit through a dummy head. Step through it.

    Visual solution and code

  6. Easy · Time O(n) · Space O(1)

    Linked List Cycle

    Detect if a linked list has a cycle.

    Answer: Detect a cycle in a linked list with O(1) space: move slow one node and fast two, and watch them meet inside the loop.

    Visual solution and code

  7. Easy · Time O(n) · Space O(1)

    Palindrome Linked List

    Check if a linked list is a palindrome.

    Answer: Check whether a linked list is a palindrome in O(1) space: find the middle, reverse the second half in place, then compare both halves.

    Visual solution and code

  8. Medium · Time O(n) · Space O(1)

    Remove Nth Node From End of List

    Remove the nth node from the end of the list.

    Answer: Remove the n-th node from the end in one pass: a dummy node, a fast pointer n + 1 ahead, and one unlink. Step through it.

    Visual solution and code

  9. Medium · Time O(n) · Space O(1)

    Reorder List

    Reorder a linked list in a specific pattern.

    Answer: Reorder a linked list to L0 → Ln → L1 → Ln-1 in O(1) space: find the middle, reverse the second half, and merge the two halves.

    Visual solution and code

  10. Medium · Time O(n) · Space O(1)

    Swap Nodes in Pairs

    Swap every two adjacent nodes.

    Answer: Swap every two adjacent nodes by changing pointers, not values: a dummy node, then three next assignments per pair.

    Visual solution and code

  11. Medium · Time O(1) · Space O(capacity)

    LRU Cache

    Design a least recently used cache with O(1) get and put, using a hash map over a doubly linked list.

    Answer: Implement an LRU cache with O(1) get and put: a hash map finds nodes, a doubly linked list keeps recency. Watch each move and eviction.

    Visual solution and code

  12. Medium · Time O(n) · Space O(1)

    Find the Duplicate Number

    Find the one repeated value in O(n) time and O(1) space by reading the array as a linked list and using Floyd's cycle detection.

    Answer: Find the duplicate in O(n) time and O(1) space: treat nums[i] as a next pointer, and Floyd's tortoise and hare finds the cycle entrance. 6 languages.

    Visual solution and code

  13. Medium · Time O(n) · Space O(n)

    Copy List with Random Pointer

    Deep-copy a linked list whose nodes also point to a random node: copy every node first, then wire next and random through a map.

    Answer: Deep-copy a linked list with random pointers: copy every node into a map, then wire next and random. Plus the O(1)-space interleaving trick. 6 languages.

    Visual solution and code

  14. Hard · Time O(n) · Space O(1)

    Reverse Nodes in k-Group

    Reverse a linked list k nodes at a time in O(1) extra space: find each group of k, reverse it in place, and relink it to the rest.

    Answer: Solve Reverse Nodes in k-Group iteratively: a dummy node, a k-step check, in-place reversal per group and careful relinking. Visualized, 6 languages.

    Visual solution and code

  15. Easy · Time O(log n) · Space O(1)

    Happy Number

    Decide whether repeatedly summing the squares of a number's digits reaches 1, using Floyd's slow and fast pointers to detect the loop.

    Answer: Solve Happy Number with Floyd's cycle detection: the digit-square sequence is a linked list, so slow and fast pointers find 1 or a loop. Code in 6 languages.

    Visual solution and code

Stack interview questions

How the stack pattern works

  1. Easy · Time O(n) · Space O(n)

    Valid Parentheses

    Determine if the input string has valid matching brackets.

    Answer: Check whether brackets are balanced and correctly nested: push every opener and match each closer against the top of the stack.

    Visual solution and code

  2. Medium · Time O(n · L) · Space O(n + L)

    Decode String

    Decode an encoded string with nested repetition rules.

    Answer: Decode strings like 3[a2[c]] with one stack of chunks: pop to the "[", read the count, and push the repeated group back.

    Visual solution and code

  3. Hard · Time O(n) · Space O(n)

    Longest Valid Parentheses

    Measure the longest stretch of a bracket string in which every parenthesis is matched.

    Answer: Find the longest well-formed parentheses substring in O(n) with a stack of indices and a -1 base, and watch each valid run get measured.

    Visual solution and code

  4. Medium · Time O(n) · Space O(n)

    Daily Temperatures

    Find how many days you have to wait for a warmer temperature.

    Answer: For each day, count the days until a warmer one: a stack of waiting days pops whenever a warmer day arrives. O(n), step by step.

    Visual solution and code

  5. Hard · Time O(n) · Space O(n)

    Largest Rectangle in Histogram

    Find the area of the largest rectangle in the histogram.

    Answer: Find the largest rectangle in a histogram in O(n): keep bars in increasing order, and measure each bar’s rectangle as it is popped.

    Visual solution and code

  6. Hard · Time O(rows · cols) · Space O(cols)

    Maximal Rectangle

    Find the largest rectangle of 1s in a binary matrix by turning each row into a histogram of column heights and solving it with a monotonic stack.

    Answer: Solve Maximal Rectangle by turning each row into a histogram of 1s and finding its largest rectangle with a monotonic stack. O(rows · cols), 6 languages.

    Visual solution and code

  7. Medium · Time O(n) · Space O(n)

    Evaluate Reverse Polish Notation

    Evaluate the value of an arithmetic expression in Reverse Polish Notation (Postfix).

    Answer: Evaluate postfix expressions with one stack: push numbers, and let each operator pop its two operands and push the result.

    Visual solution and code

  8. Medium · Time O(1) · Space O(n)

    Min Stack

    Design a stack that supports retrieving the minimum element in constant time.

    Answer: Design a stack with push, pop, top and getMin in O(1): a second stack records the minimum at every height. Duplicates included.

    Visual solution and code

  9. Medium · Time O(n log n) · Space O(n)

    Car Fleet

    Count the car fleets that reach a target by sorting cars from the front back and keeping a stack of fleet arrival times.

    Answer: Solve Car Fleet: sort cars from the front back, compute each arrival time, and push a new fleet only when a car is slower than the one ahead. 6 languages.

    Visual solution and code

  10. Easy · Time O(m + n) · Space O(n)

    Next Greater Element I

    Find the next greater element for each value of nums1 inside nums2 with one monotonic-stack pass over nums2 and a lookup map.

    Answer: Solve Next Greater Element I in O(m + n): one pass over nums2 with a decreasing stack records each value's next greater element in a map. Code in 6 languages.

    Visual solution and code

  11. Medium · Time O(n) · Space O(n)

    Next Greater Element II

    Find the next greater element of every value in a circular array with a monotonic stack of indexes and two passes over the array.

    Answer: Solve Next Greater Element II in O(n): loop i to 2n − 1, read nums[i % n], and pop a stack of waiting indexes. Visual walkthrough and code in 6 languages.

    Visual solution and code

  12. Medium · Time O(n) · Space O(n)

    Asteroid Collision

    Simulate asteroid collisions with a stack of survivors: only a left-mover meeting a right-mover on top can collide, and the smaller one explodes.

    Answer: Solve Asteroid Collision in O(n) with a stack of survivors: a left-mover pops smaller right-movers until it meets a bigger or equal one. Code in 6 languages.

    Visual solution and code

  13. Medium · Time O(n) · Space O(n)

    Online Stock Span

    Return each day's stock span in amortised O(1) with a monotonic stack of (price, span) pairs that folds smaller prices into today's span.

    Answer: Solve Online Stock Span with a monotonic stack of (price, span) pairs: each call absorbs smaller prices' spans in amortised O(1). Code in 6 languages.

    Visual solution and code

  14. Medium · Time O(n) · Space O(n)

    Remove K Digits

    Remove k digits to make the smallest number with a greedy monotonic stack: drop a digit whenever a smaller one comes after it.

    Answer: Solve Remove K Digits in O(n) with a greedy increasing stack: pop bigger digits while removals remain, trim the end, strip zeros. Code in 6 languages.

    Visual solution and code

  15. Medium · Time O(n) · Space O(n)

    Simplify Path

    Turn a Unix-style absolute path into its canonical form with a stack of directory names: skip empty parts and '.', pop on '..', push names.

    Answer: Solve Simplify Path in O(n) with a stack: split on '/', ignore empty parts and '.', pop on '..', push names, then join. Visual walkthrough, code in 6 languages.

    Visual solution and code

  16. Medium · Time O(n) · Space O(n)

    Basic Calculator II

    Evaluate an expression with + - * / and no parentheses in one pass: a stack of terms where * and / are applied to the top right away.

    Answer: Solve Basic Calculator II in O(n): push +num or -num, apply * and / to the stack top at once, then sum the stack. Visual walkthrough and code in 6 languages.

    Visual solution and code

  17. Easy · Time O(n) · Space O(n)

    Remove All Adjacent Duplicates in String

    Repeatedly remove adjacent equal letters in one pass with a stack: a letter equal to the top cancels it, otherwise it is kept.

    Answer: Solve Remove All Adjacent Duplicates in String in O(n) with a stack: pop when a letter matches the top, push otherwise. Visual walkthrough, 6 languages.

    Visual solution and code

Heap / Priority Queue interview questions

How the heap / priority queue pattern works

  1. Medium · Time O(n log k) · Space O(k)

    Kth Largest Element in an Array

    Find the kth largest element in an unsorted array.

    Answer: Find the kth largest element with a min-heap that never holds more than k values: O(n log k) time, O(k) space. Step through it.

    Visual solution and code

  2. Medium · Time O(n log k) · Space O(k)

    K Closest Points to Origin

    Find the K closest points to the origin (0, 0).

    Answer: Find the k points nearest the origin with a max-heap that evicts the farthest point once it holds more than k. O(n log k).

    Visual solution and code

  3. Medium · Time O(n) · Space O(k)

    Find K Closest Elements

    Find k closest integers to a given value x.

    Answer: Find the k values closest to x in a sorted array: shrink a window from its farther end in O(n), keeping the smaller value on a tie.

    Visual solution and code

  4. Hard · Time O(N log k) · Space O(k)

    Merge K Sorted Lists

    Merge k sorted linked lists into one sorted linked list.

    Answer: Merge k sorted linked lists by keeping each list's front node in a min-heap: pop the smallest, link it, push its next. O(N log k).

    Visual solution and code

  5. Medium · Time O(k log min(n, k)) · Space O(min(n, k))

    Kth Smallest Element in a Sorted Matrix

    Find the kth smallest value in a matrix whose rows and columns are sorted by merging the rows with a min-heap: pop k - 1 times, then read the top.

    Answer: Solve Kth Smallest Element in a Sorted Matrix with a min-heap: seed it with each row's first cell, pop k - 1 times pushing right neighbours. Binary search too.

    Visual solution and code

  6. Hard · Time O(log n) · Space O(n)

    Find Median from Data Stream

    Keep a running median of a stream with two heaps: a max-heap for the smaller half and a min-heap for the larger half.

    Answer: Maintain a running median with two heaps: a max-heap of the lower half and a min-heap of the upper half, balanced on every insert, step by step.

    Visual solution and code

  7. Easy · Time O(n log n) · Space O(n)

    Last Stone Weight

    Smash the two heaviest stones until at most one is left, using a max-heap to find the heaviest pair each round.

    Answer: Solve Last Stone Weight with a max-heap: pop the two heaviest stones, push back their difference, and repeat. Python's min-heap trick and 6 languages.

    Visual solution and code

  8. Medium · Time O(E log E) · Space O(V + E)

    Network Delay Time

    Find how long a signal takes to reach every node with Dijkstra's algorithm: a min-heap always settles the closest node next.

    Answer: Solve Network Delay Time with Dijkstra and a min-heap: pop the closest node, relax its edges, skip stale entries. Answer = max distance or -1. 6 languages.

    Visual solution and code

  9. Medium · Time O(n² log n) · Space O(n²)

    Min Cost to Connect All Points

    Connect every point at the least total Manhattan distance with Prim's algorithm: a min-heap always attaches the cheapest new point to the tree.

    Answer: Solve Min Cost to Connect All Points with Prim's algorithm and a min-heap: grow one tree, always add the cheapest new point. Dry run and code in 6 languages.

    Visual solution and code

  10. Hard · Time O(n² log n) · Space O(n²)

    Swim in Rising Water

    Find the earliest time you can swim from the top-left to the bottom-right of an elevation grid with Dijkstra, where a path costs its highest cell.

    Answer: Solve Swim in Rising Water with a min-heap: a path costs its highest elevation, so always expand the cell reachable at the lowest time. Dry run, 6 languages.

    Visual solution and code

Backtracking interview questions

How the backtracking pattern works

  1. Medium · Time O(n × 2ⁿ) · Space O(n) depth, O(n × 2ⁿ) output

    Solution Space Trees

    See the whole search space as a tree: one decision per level, and 2^n leaves that are the answers.

    Answer: Every backtracking problem walks a tree of candidates. See it on LeetCode 78: one decision per level, 2^n leaves, and where pruning would cut.

    Visual solution and code

  2. Medium · Time O(n × 2ⁿ) · Space O(n) depth, O(n × 2ⁿ) output

    Subsets (Power Set)

    Generate every subset of distinct numbers by deciding each one in turn: leave it out, take it, undo.

    Answer: Generate the power set with backtracking: decide each number in turn, record at depth n, and step through the call tree.

    Visual solution and code

  3. Medium · Time O(n · n!) · Space O(n)

    Permutations

    Generate every permutation of distinct numbers with backtracking: choose an unused number, recurse, then un-choose it so the next branch starts clean.

    Answer: Solve Permutations with backtracking: mark a number used, append, recurse, then pop and unmark. Step through the choose and un-choose tree. Code in 6 languages.

    Visual solution and code

  4. Medium · Time O(n · n!) · Space O(n)

    Permutations II

    Generate all unique permutations of numbers that may repeat: sort, then skip a value when its equal left neighbour is not yet in the path.

    Answer: Solve Permutations II: sort, then skip nums[i] when it equals nums[i − 1] and that twin is unused, so no ordering repeats. Visual walkthrough, 6 languages.

    Visual solution and code

  5. Medium · Time O(k · C(n, k)) · Space O(k)

    Combinations

    Generate every way to choose k numbers from 1 to n with backtracking that only moves forward, so each set is built exactly once.

    Answer: Solve Combinations with backtracking: choose numbers in increasing order from a start index, record paths of length k, then backtrack. Code in 6 languages.

    Visual solution and code

  6. Medium · Time O(n · 2^k) · Space O(n)

    Letter Case Permutation

    Generate every string from changing each letter's case with backtracking: each letter branches into lower and upper case, digits pass through.

    Answer: Solve Letter Case Permutation with backtracking: branch on lower and upper case for each letter, keep digits, record full strings. 2^k results, 6 languages.

    Visual solution and code

  7. Hard · Time O(n!) · Space O(n)

    N-Queens

    Place n queens on an n × n board so none attack each other with backtracking: one queen per row, and sets for columns and both diagonals.

    Answer: Solve N-Queens with backtracking: one queen per row, O(1) conflict checks with column, r − c and r + c sets. Visual board walkthrough, 6 languages.

    Visual solution and code

  8. Medium · Time O(n · 2^n) · Space O(n)

    Subsets II

    List every subset of an array that may contain duplicates, each exactly once: sort, then skip a repeated value at the same level.

    Answer: Solve Subsets II with backtracking: sort, record every path, and skip a value already tried at the same level so no subset repeats. Visualized, 6 languages.

    Visual solution and code

  9. Medium · Time O(n^(T/m + 1)) · Space O(T/m) depth

    Combination Sum

    Find every combination that sums to a target, reusing numbers freely, and prune with a sorted break.

    Answer: Find every combination that sums to a target with reuse allowed. Sort, recurse on the same index, and break early — step through it.

    Visual solution and code

  10. Medium · Time O(n · 2^n) · Space O(n)

    Combination Sum II

    Find every unique combination that adds up to a target, using each number once: sort, skip repeats on the same level, and break early.

    Answer: Solve Combination Sum II with backtracking: sort, use each number once, and skip a value already tried at the same level so no combination repeats. 6 languages.

    Visual solution and code

  11. Medium · Time O(4ⁿ / √n) · Space O(n) depth, O(4ⁿ / √n) output

    Generate Parentheses

    Build only well-formed strings of n pairs, using two guards instead of generating and filtering.

    Answer: Generate every well-formed string of n pairs with backtracking: open while under n, close while under open. Step through the call tree.

    Visual solution and code

  12. Medium · Time O(m × n × 3ᴸ) · Space O(L) stack

    Word Search

    Find whether a word can be traced through adjacent cells of a letter grid, marking each cell on the way down and restoring it on the way back.

    Answer: Solve Word Search with backtracking: mark a cell, try its four neighbours, write the letter back. Step through the board.

    Visual solution and code

  13. Medium · Time O(4^n · n) · Space O(n)

    Letter Combinations of a Phone Number

    List every letter string a phone number's digits can spell with a backtracking DFS: choose a letter, recurse, undo.

    Answer: Solve Letter Combinations of a Phone Number with backtracking: one letter per digit, choose, recurse, undo. Watch the keys cycle like an odometer. 6 languages.

    Visual solution and code

  14. Medium · Time O(n · 2^n) · Space O(n)

    Palindrome Partitioning

    List every way to cut a string into palindromes with backtracking: cut a palindrome off the front, partition the rest, undo.

    Answer: Solve Palindrome Partitioning with backtracking: try every palindrome prefix, partition the rest, then undo. Step through the cuts, with code in 6 languages.

    Visual solution and code

Graphs (Topological Sort) interview questions

How the graphs (topological sort) pattern works

  1. Medium · Time O(V + E) · Space O(V + E)

    Course Schedule

    Decide whether every course can be finished, by counting how many Kahn's algorithm can take before a prerequisite cycle stops it.

    Answer: Solve Course Schedule with Kahn's algorithm: count prerequisites, take the courses that reach zero, and spot the cycle. Step through it.

    Visual solution and code

  2. Medium · Time O(V + E) · Space O(V + E)

    Course Schedule II

    Return an order in which every course can be taken — Kahn's algorithm writes it out, and a prerequisite cycle empties it.

    Answer: Return a valid order for Course Schedule II with Kahn's algorithm, and an empty array when a prerequisite cycle makes it impossible.

    Visual solution and code

  3. Hard · Time O(C) · Space O(U + E)

    Alien Dictionary

    Recover the letter order of an alien alphabet from a sorted word list: compare neighbouring words for facts, then topologically sort the letters.

    Answer: Solve Alien Dictionary: compare adjacent words for letter-order facts, then run Kahn's algorithm on the letters, detecting cycles.

    Visual solution and code

Dynamic Programming interview questions

How the dynamic programming pattern works

  1. Easy · Time O(n) · Space O(1)

    Fibonacci Number

    Compute the nth Fibonacci number bottom-up in O(n) time and O(1) space by keeping only the last two values, instead of exponential recursion.

    Answer: Solve Fibonacci Number in O(n) time and O(1) space: see why plain recursion is exponential, then build up with two variables. Visual walkthrough, 6 languages.

    Visual solution and code

  2. Easy · Time O(n) · Space O(1)

    Counting Bits

    Given an integer n, return an array of the number of 1's for each number from 0 to n.

    Answer: Count the 1 bits of every number up to n in O(n) using dp[i] = dp[i >> 1] + (i & 1). Step through the shift.

    Visual solution and code

  3. Easy · Time O(n²) · Space O(n²)

    Pascal's Triangle

    Generate the first numRows rows of Pascal's triangle bottom-up: every row starts and ends with 1, and each inside value is the sum of the two above.

    Answer: Solve Pascal's Triangle with simple DP: each row starts and ends with 1, and row[j] = prev[j - 1] + prev[j]. Visual walkthrough and code in 6 languages.

    Visual solution and code

  4. Easy · Time O(n) · Space O(n)

    Min Cost Climbing Stairs

    Find the cheapest way to the top of a staircase where each step has a cost, with a one-line DP over the two ways into every step.

    Answer: Solve Min Cost Climbing Stairs with DP: dp[i] = min(dp[i-1] + cost[i-1], dp[i-2] + cost[i-2]), and the top is one past the last stair. Code in 6 languages.

    Visual solution and code

  5. Medium · Time O(n) · Space O(n)

    House Robber

    Maximise the money robbed from a row of houses without robbing two adjacent ones, with a one-dimensional DP.

    Answer: Solve House Robber with one recurrence, dp[i] = max(dp[i-1], dp[i-2] + nums[i-1]), and watch the table fill and the robbed houses traced back.

    Visual solution and code

  6. Medium · Time O(n) · Space O(1)

    House Robber II

    Rob houses arranged in a circle, where the first and last are neighbours, by running House Robber twice on two straight streets.

    Answer: Solve House Robber II: the first and last houses are neighbours, so run House Robber on houses 0..n-2 and 1..n-1 and take the larger.

    Visual solution and code

  7. Medium · Time O(n) · Space O(h)

    House Robber III

    Rob a binary tree of houses without robbing two linked ones: a post-order DFS returns, for every house, the best total if you rob it and if you skip it.

    Answer: Solve House Robber III with tree DP: each node returns (rob, skip). rob = val + children skipped, skip = best of each child. Dry run and code in 6 languages.

    Visual solution and code

  8. Medium · Time O(amount · k) · Space O(amount)

    Coin Change

    Find the fewest coins that make up an amount, or -1 if it cannot be made, with a bottom-up DP over amounts.

    Answer: Find the fewest coins for an amount with dp[a] = min(dp[a - coin] + 1), see why greedy fails, and watch the table fill and the answer trace back.

    Visual solution and code

  9. Medium · Time O(amount · coins) · Space O(amount)

    Coin Change II

    Count the combinations of coins that make an amount with a 1D DP, looping over coins outside so each combination is counted once.

    Answer: Solve Coin Change II with a 1D DP: dp[a] += dp[a − coin], coins in the outer loop so 1+2 and 2+1 count once. Visualized, with code in 6 languages.

    Visual solution and code

  10. Medium · Time O(n) · Space O(1)

    Maximum Product Subarray

    Find the largest product of a contiguous subarray in one pass by tracking both the largest and the smallest running product.

    Answer: Solve Maximum Product Subarray in O(n): track the largest and smallest product ending here, swap them on a negative, and watch each step.

    Visual solution and code

  11. Medium · Time O(target · n) · Space O(target)

    Combination Sum IV

    Count the ordered sequences of numbers that add up to a target with a one-dimensional DP over every total.

    Answer: Solve Combination Sum IV with a 1D DP: dp[t] adds dp[t - num] for every num that can come last. Watch the table fill step by step.

    Visual solution and code

  12. Medium · Time O(n · sum) · Space O(sum)

    Partition Equal Subset Sum

    Decide whether an array splits into two halves with equal sums, using a 0/1 knapsack row of reachable sums filled right to left.

    Answer: Solve Partition Equal Subset Sum with a 1D 0/1 knapsack: track which sums are reachable and sweep right to left so each number is used once. 6 languages.

    Visual solution and code

  13. Medium · Time O(n · sum) · Space O(sum)

    Target Sum

    Count the ways to put + and − in front of each number to reach a target, by turning it into counting subsets with sum (total + target) / 2.

    Answer: Solve Target Sum by turning signs into a subset-sum count: the plus group must add up to (total + target) / 2. 1D 0/1 knapsack, code in 6 languages.

    Visual solution and code

  14. Medium · Time O(n) · Space O(n)

    Decode Ways

    Determine the total number of ways to decode a string of digits into letters (A-Z).

    Answer: Count decodings of a digit string with dp[i] = dp[i-1] + dp[i-2] under two rules, and see why a zero is never a letter.

    Visual solution and code

  15. Medium · Time O(m · n) · Space O(m · n)

    Maximal Square

    In an m x n grid of 0s and 1s, find the biggest square made entirely of 1s and return how many cells it covers.

    Answer: Find the largest all-ones square with dp[i][j] = min(top, left, diagonal) + 1, and see why it is a minimum and why the answer is an area.

    Visual solution and code

  16. Medium · Time O(m · n) · Space O(m · n)

    Unique Paths

    Count the distinct routes a robot can take from the top-left cell of an m x n grid to the bottom-right cell when every step goes one cell down or one cell right.

    Answer: Count a robot’s right-and-down paths across an m by n grid with dp[i][j] = dp[i-1][j] + dp[i][j-1]. Step through the fill.

    Visual solution and code

  17. Medium · Time O(m · n) · Space O(m · n)

    Unique Paths II

    Count right-and-down paths through a grid with obstacles using DP: an obstacle holds 0 paths, every open cell adds the paths from above and from the left.

    Answer: Solve Unique Paths II with grid DP: obstacles hold 0 paths, open cells add the counts from above and from the left. Visual walkthrough, code in 6 languages.

    Visual solution and code

  18. Medium · Time O(m · n) · Space O(m · n)

    Minimum Path Sum

    Find the cheapest top-left to bottom-right path in a grid moving only right or down, with a DP table where each cell takes the cheaper of above and left.

    Answer: Solve Minimum Path Sum with grid DP: each cell costs its value plus the cheaper of above and left; edges have one way in. Code in 6 languages.

    Visual solution and code

  19. Medium · Time O(n²) · Space O(n)

    Triangle

    Find the minimum path sum from the top of a triangle to the bottom with bottom-up DP: each cell adds the cheaper of its two children.

    Answer: Solve Triangle bottom-up: start from the last row and set each cell to its value plus the cheaper child below. One array, O(n) space. Dry run, 6 languages.

    Visual solution and code

  20. Medium · Time O(m · n) · Space O(m · n)

    Longest Common Subsequence

    Find the length of the longest common subsequence of two strings with a 2D DP table: extend the diagonal on a match, otherwise take the better of up and left.

    Answer: Solve Longest Common Subsequence with a 2D DP table: match gives diagonal + 1, otherwise max of up and left. See the table fill and the LCS rebuild.

    Visual solution and code

  21. Medium · Time O(m · n) · Space O(m · n)

    Edit Distance

    Find the minimum number of inserts, deletes and replaces to turn one word into another with a 2D DP table, and rebuild the edits from it.

    Answer: Solve Edit Distance with a 2D DP table: a match is free, otherwise 1 + min(replace, delete, insert). Watch the table fill and the edits rebuild. 6 languages.

    Visual solution and code

  22. Medium · Time O(m · n) · Space O(m · n)

    Interleaving String

    Decide whether s3 is s1 and s2 woven together with a 2D DP table: each cell asks whether two prefixes can build a prefix of s3.

    Answer: Solve Interleaving String with a 2D DP table: dp[i][j] is true when s1[:i] and s2[:j] build s3[:i+j]. Why greedy fails, a dry run, and code in 6 languages.

    Visual solution and code

  23. Hard · Time O(m · n) · Space O(m · n)

    Distinct Subsequences

    Count the ways t appears as a subsequence of s with a 2D DP table: every letter of s is skipped, or used when it matches the next letter of t.

    Answer: Solve Distinct Subsequences with DP: dp[i][j] = dp[i-1][j] + dp[i-1][j-1] when the letters match. Why column 0 is 1, a dry run, and code in 6 languages.

    Visual solution and code

  24. Hard · Time O(m · n) · Space O(m · n)

    Regular Expression Matching

    Match a string against a pattern with '.' and '*' using a 2D DP table: a star pair matches zero copies or eats one more letter.

    Answer: Solve Regular Expression Matching with a 2D DP table: '.' matches any letter, 'x*' matches zero copies or one more. Dry run, edge cases, 6 languages.

    Visual solution and code

  25. Hard · Time O(m · n) · Space O(m · n)

    Wildcard Matching

    Match a string against a pattern with '?' and '*' using a 2D DP table: a star matches the empty sequence or eats one more letter.

    Answer: Solve Wildcard Matching with a 2D DP table: '?' matches one letter, '*' matches the empty sequence or one more letter. Dry run, edge cases, 6 languages.

    Visual solution and code

  26. Hard · Time O(n³) · Space O(n²)

    Burst Balloons

    Maximise the coins from bursting every balloon with interval DP: guess which balloon in a range bursts last, so its neighbours are the range's walls.

    Answer: Solve Burst Balloons with interval DP: pick the last balloon k in (l, r), earn vals[l]·vals[k]·vals[r] plus both sides. Why last, not first, plus 6 languages.

    Visual solution and code

  27. Medium · Time O(n) · Space O(1)

    Best Time to Buy and Sell Stock with Cooldown

    Maximise stock profit when every sale is followed by a one-day cooldown, with three DP states per day: hold, sold and rest.

    Answer: Solve Best Time to Buy and Sell Stock with Cooldown with three states: hold, sold, rest. Each day's transitions, a dry run and O(1)-space code in 6 languages.

    Visual solution and code

  28. Medium · Time O(n²) · Space O(n)

    Longest Increasing Subsequence

    Find how long the longest strictly increasing subsequence of the integer array nums is; its elements do not have to sit next to each other.

    Answer: Solve LIS with dp[i] = the longest run ending at i, and see why the answer is max(dp) and not the last cell.

    Visual solution and code

  29. Medium · Time O(n² · k) · Space O(n + d)

    Word Break

    Decide whether the string s can be cut, end to end, into pieces that each appear in a given list of dictionary words, where any word may be used more than once.

    Answer: Decide if a string splits into dictionary words with dp over prefixes: dp[i] is true when some reachable j leaves a word behind.

    Visual solution and code

  30. Hard · Time O(n log n) · Space O(n)

    Maximum Profit in Job Scheduling

    Given n jobs where every job has a start time, end time, and profit, return the maximum profit without overlapping jobs.

    Answer: Maximise profit over non-overlapping jobs: sort by end time, binary search the last compatible job, take or leave each one.

    Visual solution and code

Greedy Algorithms interview questions

How the greedy algorithms pattern works

  1. Easy · Time O(n) · Space O(1)

    Best Time to Buy and Sell Stock

    Find the best profit from one buy followed by one later sell.

    Answer: Track the cheapest price so far and the best sale so far in one pass: O(n) time, O(1) space. Step through it.

    Visual solution and code

  2. Medium · Time O(n) · Space O(1)

    Best Time to Buy and Sell Stock II

    Maximise profit with unlimited stock transactions greedily: add every day-to-day price rise, since any climb is the sum of its daily gains.

    Answer: Solve Best Time to Buy and Sell Stock II in O(n) greedily: add every positive day-to-day difference. See why it equals the best set of trades. 6 languages.

    Visual solution and code

  3. Medium · Time O(n) · Space O(1)

    Gas Station

    Find the starting gas station to complete the circuit.

    Answer: Find the station that completes the circuit in one pass with two running tanks: O(n) time, O(1) space. Step through it.

    Visual solution and code

  4. Medium · Time O(n) · Space O(1)

    Jump Game

    Determine if you can reach the last index.

    Answer: Decide if you can reach the last index by tracking the furthest reachable index in one pass. See why a single number is enough.

    Visual solution and code

  5. Medium · Time O(n) · Space O(1)

    Jump Game II

    Find the fewest jumps to the last index in one greedy pass: each jump covers a window, and the next window ends at the farthest reach.

    Answer: Solve Jump Game II in O(n) time and O(1) space: treat each jump as a BFS window and jump when i reaches the window's end. Step through it in 6 languages.

    Visual solution and code

  6. Medium · Time O(n) · Space O(1)

    Task Scheduler

    Find the fewest CPU intervals to run every task with n idle gaps between repeats, from a frame set by the most frequent task.

    Answer: Solve Task Scheduler in O(n): the most frequent task builds a frame of (maxCount − 1) · (n + 1) + ties slots, and the rest fill its gaps. 6 languages.

    Visual solution and code

  7. Medium · Time O(n) · Space O(1)

    Partition Labels

    Split a string into as many parts as possible so each letter appears in only one part, by cutting when every letter seen has ended.

    Answer: Solve Partition Labels in O(n): record each letter's last index, stretch the part's end as you scan, and cut when i reaches it. Visualized, 6 languages.

    Visual solution and code

  8. Medium · Time O(n) · Space O(1)

    Valid Parenthesis String

    Check a string of '(', ')' and '*' in one greedy pass by tracking the lowest and highest number of open brackets any choice could give.

    Answer: Solve Valid Parenthesis String in O(n) time and O(1) space: track the range [lo, hi] of possible open brackets, since each * can be (, ) or empty. 6 languages.

    Visual solution and code

  9. Medium · Time O(n log n) · Space O(n)

    Hand of Straights

    Decide whether cards can be split into groups of consecutive values: the smallest card left must start a group, so build groups greedily from it.

    Answer: Solve Hand of Straights greedily: the smallest card left must begin a group, so take groupSize consecutive values from it using a sorted count map. 6 languages.

    Visual solution and code

  10. Medium · Time O(1) · Space O(1)

    Integer to Roman

    Convert an integer to a Roman numeral greedily: walk a 13-entry value table, subtraction pairs included, and write each symbol while it fits.

    Answer: Solve Integer to Roman greedily: walk 13 values from 1000 down to 1, including CM, CD, XC, XL, IX and IV, writing each while it fits. Code in 6 languages.

    Visual solution and code

Trie interview questions

How the trie pattern works

  1. Medium · Time O(L) · Space O(N·L)

    Implement Trie (Prefix Tree)

    Design a data structure that supports insert, search, and startsWith.

    Answer: Implement insert, search and startsWith for a trie. Step through node creation and path walks, and see why search checks the end-of-word flag.

    Visual solution and code

  2. Medium · Time O(N) · Space O(total letters)

    Design Add and Search Words Data Structure

    Store words in a trie and search them with '.' wildcards using a DFS that branches at every dot.

    Answer: Solve Design Add and Search Words with a trie: addWord inserts, search runs a DFS that tries every child at a '.' and backs up.

    Visual solution and code

  3. Hard · Time O(m · n · 3^L) · Space O(total letters)

    Word Search II

    Find every word from a list on a letter grid with one backtracking DFS per cell, guided by a trie of the words.

    Answer: Solve Word Search II with a trie: one DFS from each cell follows only prefixes some word has, records words as it reaches them.

    Visual solution and code

Prefix Sum interview questions

How the prefix sum pattern works

  1. Medium · Time O(N + Q) · Space O(N + Q)

    Count Vowels in Ranges

    Answer range queries counting the words that start and end with a vowel — one pass to build a prefix count, then one subtraction per query.

    Answer: Mark each word that starts and ends with a vowel, build a prefix count, and answer every range query with one subtraction.

    Visual solution and code

  2. Medium · Time O(n) · Space O(1)

    Product of Array Except Self

    Return the product of every element except the current one, without division, using prefix and suffix products.

    Answer: Solve Product of Array Except Self without division: a left pass of prefix products and a right pass of suffix products in one output array.

    Visual solution and code

  3. Medium · Time O(N) · Space O(N)

    Subarray Sum Equals K

    Count the contiguous subarrays whose sum equals k in one pass, using a running prefix sum and a hash map of the sums seen so far.

    Answer: Count subarrays that sum to k in one pass: keep a running sum and look up how many earlier prefix sums equal sum - k. Step through it.

    Visual solution and code

Matrices interview questions

How the matrices pattern works

  1. Medium · Time O(m·n) · Space O(1)

    Spiral Matrix

    Return all elements of the matrix in spiral order.

    Answer: Solve Spiral Matrix by shrinking four boundaries layer by layer. Step through every pass of the walk, with the.

    Visual solution and code

  2. Medium · Time O(n²) · Space O(1)

    Rotate Image

    Rotate the matrix by 90 degrees clockwise in-place.

    Answer: Rotate an n × n matrix 90° clockwise in place: transpose across the diagonal, then reverse every row. Step through both phases.

    Visual solution and code

  3. Medium · Time O(m·n) · Space O(1)

    Set Matrix Zeroes

    Spread every 0 in a matrix across its full row and column, in place.

    Answer: Zero every row and column that holds a zero, in place, using row 0 and column 0 as markers. Step through each pass with the.

    Visual solution and code

Bit Manipulation interview questions

How the bit manipulation pattern works

  1. Easy · Time O(32) · Space O(1)

    Number of 1 Bits

    Count the set bits of an integer by clearing the lowest one with n & (n - 1) until nothing is left.

    Answer: Count set bits with Brian Kernighan's trick: n & (n - 1) clears the lowest 1, so the loop runs once per set bit. Watch each bit clear.

    Visual solution and code

  2. Easy · Time O(n) · Space O(1)

    Single Number

    Find the one number that appears once when every other appears twice, in O(n) time and O(1) space with XOR.

    Answer: Solve Single Number in O(n) time and O(1) space: XOR every value so each pair cancels, leaving the one that appears once.

    Visual solution and code

  3. Easy · Time O(n) · Space O(1)

    Missing Number

    Find the one number missing from 0..n in O(n) time and O(1) space by XORing every index and value so the pairs cancel.

    Answer: Solve Missing Number in O(n) time and O(1) space: XOR every index and value so each pair cancels and only the missing number is left.

    Visual solution and code

  4. Easy · Time O(32) · Space O(1)

    Reverse Bits

    Reverse the 32 bits of an unsigned integer by shifting each bit off n and onto the result.

    Answer: Reverse the bits of a 32-bit integer: shift the result left, copy n & 1 into it, shift n right, 32 times. Watch each bit cross over.

    Visual solution and code

  5. Medium · Time O(32) · Space O(1)

    Sum of Two Integers

    Add two integers without the + or - operators: XOR adds without carrying, and AND shifted left is the carry.

    Answer: Add two integers without +: a ^ b adds each column without the carry, (a & b) << 1 is the carry, repeat until it is 0. Step through each round.

    Visual solution and code

  6. Medium · Time O(log n) · Space O(1)

    Pow(x, n)

    Compute x to the power n in O(log n) by squaring: read n in binary and multiply in x^1, x^2, x^4, ... for every 1 bit.

    Answer: Solve Pow(x, n) in O(log n) with exponentiation by squaring: square the base each round, multiply it in on 1 bits, flip negative n. Code in 6 languages.

    Visual solution and code

  7. Medium · Time O(log |x|) · Space O(1)

    Reverse Integer

    Reverse the digits of a 32-bit integer by popping the last digit and pushing it onto the result, checking for overflow before each push.

    Answer: Solve Reverse Integer by popping digits with % 10 and pushing them with × 10, checking INT_MAX / 10 before each push so nothing overflows. 6 languages.

    Visual solution and code