17. Valid Palindrome

easyAsked at Zoom

Determine if a string is a palindrome considering only alphanumerics.

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 to lowercase and removing non-alphanumeric characters. Spaces, punctuation, and case must be ignored.

Constraints

1 <= s.length <= 2 * 10^5
ASCII printable 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. Normalize then reverse

Filter to alphanumerics, lowercase, reverse and compare.

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

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

Tradeoff:

2. Two-pointer in-place

Walk from both ends, skipping non-alphanumeric, comparing lowercase chars.

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

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

Tradeoff:

Zoom-specific tips

Zoom values minimal-allocation string parsing because chat messages and meeting topics are validated on hot paths — show the two-pointer version first.

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Output

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