12. Valid Palindrome

easyAsked at Squarespace

Decide whether a string is a palindrome after stripping non-alphanumeric characters and lowercasing; Squarespace uses it to gauge two-pointer fluency on cleaned input.

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 it to lowercase and removing all non-alphanumeric characters. An empty string counts as 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

Build a cleaned lowercase string then compare it to its reverse.

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

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

Tradeoff:

2. Two pointers, skip non-alnum

Walk left and right inward, skipping non-alphanumerics and comparing lowercase chars. Constant extra memory.

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:

Squarespace-specific tips

Squarespace evaluators want you to handle empty strings up front and to note that the same cleaning logic shows up when canonicalizing user-typed URL slugs at publish time.

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Output

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