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55. Simplify Path

mediumAsked at Reddit

Simplify a Unix-style absolute file path. Reddit uses this string + stack problem to test parsing — the same pattern they apply when canonicalizing /r/sub/comments/abc/comment/xyz URLs for cache keying.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Source citations

Public interview reports confirming this problem appears in Reddit loops.

  • Glassdoor (2026-Q1)— Reddit phone screen, common stack/string problem.

Problem

Given a string path, which is an absolute path (starting with a slash '/') to a file or directory in a Unix-style file system, convert it to the simplified canonical path. In a Unix-style file system: '.' refers to the current directory; '..' refers to the directory up a level; any multiple consecutive slashes (i.e. '//') are treated as a single slash '/'. The canonical path should have format: starts with '/', no trailing '/', single slashes between components, no '.' or '..'.

Constraints

  • 1 <= path.length <= 3000
  • path consists of English letters, digits, period '.', slash '/' or '_'.
  • path is a valid absolute Unix path.

Examples

Example 1

Input
path = "/home/"
Output
"/home"

Example 2

Input
path = "/../"
Output
"/"

Example 3

Input
path = "/home//foo/"
Output
"/home/foo"

Example 4

Input
path = "/a/./b/../../c/"
Output
"/c"

Approaches

1. Repeated regex replace

Repeatedly replace /./ -> /, /xxx/../ -> /, // -> /.

Time
O(n^2)
Space
O(n)
// Anti-pattern: regex replace until stable.

Tradeoff: Quadratic and fragile (regex order matters).

2. Split + stack (optimal)

Split on '/'. For each token: skip empty/'.', pop on '..', else push. Rejoin.

Time
O(n)
Space
O(n)
function simplifyPath(path) {
  const stack = [];
  for (const token of path.split('/')) {
    if (token === '' || token === '.') continue;
    if (token === '..') stack.pop();
    else stack.push(token);
  }
  return '/' + stack.join('/');
}

Tradeoff: Single pass with a stack. Reddit's URL canonicalization library uses this exact pattern.

Reddit-specific tips

Reddit interviewers grade on the stack reflex and on handling all four cases (empty, '.', '..', name). Bonus signal: connect to comment-tree URL canonicalization where each segment is a comment slug.

Common mistakes

  • Popping when stack is empty (in JS, .pop on empty is undefined; harmless but check requirements).
  • Forgetting the leading slash in the result.
  • Using string concatenation in a loop (O(n^2) in some languages).

Follow-up questions

An interviewer at Reddit may pivot to one of these next:

  • URL encode/decode.
  • Longest absolute file path (LC 388).
  • Mini parser (LC 385) — similar token-based parsing.

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Output

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FAQ

What if '..' is at the root?

Stack is empty; pop is a no-op. Result remains '/'.

Are there variant separators?

Unix only. For Windows-style, you'd normalize backslashes too.

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