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.
Problem Understanding
Simplify Path: given an absolute Unix-style path, return its canonical form. . means the current directory, .. the parent, and any run of slashes counts as one. The result starts with a single /, has no trailing slash, and contains no . or ...
Example: "/home/user/Documents/../Pictures" returns "/home/user/Pictures".
The Intuition: Where Am I Now?
Read the path left to right as a series of moves, keeping a stack of the directories you are inside:
- Empty part or
.: you have not moved; ignore it. Empty parts come from//, the leading/and a trailing/. ..: go up a level; pop, unless you are already at the root.- Anything else: a directory name, even
...; push it.
At the end the stack is the canonical path: join it with / and put a / in front. The same push-and-pop bookkeeping drives Valid Parentheses and Decode String.
Interactive Walkthrough
The parts of path.split('/') are on top, with empty parts drawn as "". Below them are the directories on the stack. When .. leaves a directory it turns red for one step. Try Above root to see .. ignored at the root.
Dry Run Table
Input: "/.../a/../b/c/../d/./"
| Part | Action | Stack after |
|---|---|---|
| "" | ignore | [] |
| ... | push | [...] |
| a | push | [..., a] |
| .. | pop a | [...] |
| b | push | [..., b] |
| c | push | [..., b, c] |
| .. | pop c | [..., b] |
| d | push | [..., b, d] |
| . | ignore | [..., b, d] |
| "" | ignore | [..., b, d] |
Answer: "/.../b/d".
The Solution Template
Split, then a stack of names. The C version uses strtok, which already skips empty parts.
Simplify Path Code
1var simplifyPath = function(path) {2 const stack = [];3 for (const part of path.split('/')) {4 if (part === '' || part === '.') continue;5 if (part === '..') {6 stack.pop();7 } else {8 stack.push(part);9 }10 }11 return '/' + stack.join('/');12};
Edge Cases & Common Mistakes
..at the root: ignore it;"/../"is"/".- Names made of dots:
...is a normal directory name; only.and..are special. - Multiple slashes:
"/a//b"splits into an empty part between the slashes; skip it. - Empty result: join gives an empty string; the answer is
"/", not"". - JavaScript
pop()on an empty array: returnsundefinedand changes nothing, so no guard is needed there.
Summary
Split the path on '/'. Ignore empty parts and '.', pop on '..' when the stack is not empty, and push every other name. Return '/' plus the stack joined with '/'. O(n) time, O(n) space.
More practice: DSA interview questions with answers · patterns cheat sheet