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53. Simplify Path

mediumAsked at Datadog

Convert a Unix-style absolute path into its canonical form. Datadog uses this for the stack-based path-segment pattern — same shape as their tag-prefix normalization for hierarchical metric names.

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

Source citations

Public interview reports confirming this problem appears in Datadog loops.

  • Glassdoor (2026-Q1)— Datadog backend infra onsite.

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, a period '.' refers to the current directory, a double period '..' refers to the directory up a level, and any multiple consecutive slashes are treated as a single slash '/'. The canonical path should: start with a single '/', not have any '.' or '..' or trailing '/'.

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. String replacement loop

Repeatedly replace '/./', '//', and 'foo/../' until stable.

Time
O(n^2)
Space
O(n)
// Loop: replace '//' with '/', replace '/./' with '/', replace '/foo/../' with '/'.
// Quadratic and fragile.

Tradeoff: Pattern-based; brittle on edge cases. Datadog will fail this.

2. Split + stack (optimal)

Split on '/'. For each segment: empty or '.' = skip; '..' = pop; else push. Join with '/' and prefix with '/'.

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: Single pass, clean. Datadog-canonical: the segment-stack is the same primitive they use for hierarchical-tag normalization.

Datadog-specific tips

Datadog grades on whether you recognize the stack model: directories are LIFO; '..' is pop, others are push, '.' and empty are no-ops. Articulate this before coding. Bonus: discuss what happens on a relative path (you'd seed the stack with the cwd).

Common mistakes

  • Popping an empty stack on '..' at the root — silently OK in JS (returns undefined), but the canonical form is '/'.
  • Forgetting to filter empty segments — '/home//foo' would give '//home//foo'.
  • Joining without the leading '/' — produces 'home/foo' instead of '/home/foo'.

Follow-up questions

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

  • Validate path syntax — additional checks.
  • Resolve relative paths — start with current dir as the initial stack.
  • Datadog-style: canonicalize hierarchical metric names like 'aws.ec2..cpu' -> 'aws.ec2.cpu'.

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Output

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FAQ

Why a stack?

Directories are LIFO: '..' goes back one level. A stack models the directory chain naturally.

What if '..' at the root?

Pop on an empty stack is a no-op in JS (Array.pop returns undefined). The result remains the root, which is the correct behavior — you can't go above /.

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