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53. Simplify Path

mediumAsked at Vercel

Given an absolute Unix file path, simplify it (canonical form). Vercel asks this directly — they literally need this logic in their edge router for canonicalizing URL paths and resolving `..` segments.

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

Source citations

Public interview reports confirming this problem appears in Vercel loops.

  • Glassdoor (2025-Q4)— Vercel routing-team onsite; literally their bread and butter.
  • Blind (2026-Q1)— Reported as the most likely Vercel-specific 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. The canonical path should have the following format: starts with /, no trailing /, no '.' or '..', and no multiple consecutive /.

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. Regex normalization passes

Repeatedly apply regex substitutions for // and .. until stable.

Time
O(n^2) worst case
Space
O(n)
// Tempting but error-prone — `..` with no parent, edge cases like `/...` (valid filename) break.

Tradeoff: Hard to get right. The stack version is the textbook answer.

2. Stack with split (optimal)

Split on '/'. Walk segments: '' and '.' skip. '..' pops if non-empty. Else push. Join with '/' and prepend.

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

Tradeoff: Five-line solution. The split handles consecutive slashes by producing empty strings, which are skipped. Pop on empty stack is a no-op in JS — already correct for `/..`.

Vercel-specific tips

Vercel grades this on whether you reach for split + stack immediately. Bonus signal: noting that '...' or '....' are VALID filenames (don't treat them as pseudo-parent), and that `stack.pop()` on empty is safely a no-op in JS. They may follow up with relative paths (no leading slash) or Windows-style backslashes.

Common mistakes

  • Treating '...' as '..' — wrong, it's a regular filename.
  • Using replace on // — works but doesn't handle the `..` walk.
  • Forgetting to prepend the leading '/' — `/home` not `home`.

Follow-up questions

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

  • Simplify a relative path (no leading slash).
  • Add support for `~/` expansion to a home directory.
  • Streaming variant — segments arrive one at a time.

Solve it now

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Output

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FAQ

Why split + stack?

split('/') turns the path into a list of segments; the stack naturally models 'go up one level' (pop) and 'go into a directory' (push). Both empty segments (consecutive slashes) and '.' are no-ops.

What about '/...'?

Three dots is a regular filename, not a special token. Only '.' (current) and '..' (parent) are special. Your code should push '...' normally.

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