13. Valid Palindrome

easyAsked at Yelp

Determine whether a string is a palindrome after stripping non-alphanumerics and lower-casing — Yelp uses this to test two-pointer fluency before scaling to review-fraud near-duplicate detection.

By Alex Chen, Founder, InterviewChamp.AI · Last verified 2026-05-20

Problem

Given a string s, return true if it is a palindrome after converting all uppercase letters to lowercase and removing all non-alphanumeric characters. An empty string is a palindrome.

Constraints

1 <= s.length <= 2 * 10^5
s consists only of printable ASCII characters

Examples

Example 1

Input

s = "A man, a plan, a canal: Panama"

Output

true

Example 2

Input

s = "race a car"

Output

false

Approaches

1. Clean and reverse

Filter then compare the cleaned string to its reverse.

Time: O(n)
Space: O(n)

const c = s.toLowerCase().replace(/[^a-z0-9]/g, '');
return c === c.split('').reverse().join('');

Tradeoff:

2. Two pointers

Walk inward from both ends, skipping non-alphanumerics and comparing case-insensitively.

Time: O(n)
Space: O(1)

function isPalindrome(s) {
  const ok = (c) => /[a-z0-9]/i.test(c);
  let l = 0, r = s.length - 1;
  while (l < r) {
    while (l < r && !ok(s[l])) l++;
    while (l < r && !ok(s[r])) r--;
    if (s[l].toLowerCase() !== s[r].toLowerCase()) return false;
    l++; r--;
  }
  return true;
}

Tradeoff:

Yelp-specific tips

Yelp interviewers will pivot to review fraud — be ready to discuss how a normalized-text comparison generalizes to detecting near-duplicate spam reviews across business listings.

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Output

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