What is the Subarray Sum Equals K pattern?

The Subarray Sum Equals K is a common coding interview pattern in Prefix Sum problems. It provides a structured approach to solving related algorithm challenges efficiently.

When should I use Subarray Sum Equals K?

Use the Subarray Sum Equals K pattern when you encounter problems matching its characteristics. Check the "When to Use" and "Clues" sections in the lesson for specific indicators.

Is Subarray Sum Equals K asked in FAANG interviews?

Yes, Subarray Sum Equals K is an essential pattern commonly tested in FAANG and top tech company technical interviews.

Prefix Sum

Medium

Subarray Sum Equals K

Find number of subarrays with sum K.

10 min read

Solve on LeetCode

Problem Understanding

Given an array of integers nums and an integer k, return the total number of subarrays whose sum equals to k.

Strategy (HashMap + Prefix Sum)

We want Sum(i, j) == k.
Since Sum(i, j) = Prefix[j] - Prefix[i-1], we want:
Prefix[j] - Prefix[i-1] == k
=> Prefix[i-1] == Prefix[j] - k.

Algorithm:

Iterate through array, maintaining current_sum.
Check HashMap: how many times have we seen current_sum - k before?
Add that count to our answer.
Add current_sum to the HashMap.

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