17. Valid Palindrome

easyAsked at Ola

Determine if a phrase reads the same forward and backward, ignoring non-alphanumerics.

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

Problem

A phrase is a palindrome if after converting all uppercase letters to lowercase and removing all non-alphanumeric characters it reads the same forward and backward. Given a string s, return true if it is a palindrome, false otherwise.

Constraints

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

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 then reverse

Filter to alphanumerics, lowercase, and compare to its reverse.

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

const t = s.toLowerCase().replace(/[^a-z0-9]/g,'');
return t === [...t].reverse().join('');

Tradeoff:

2. Two pointers

Walk lo and hi inward skipping non-alphanumeric. O(n) time and O(1) extra space.

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:

Ola-specific tips

Ola uses two-pointer palindrome to check linear-time discipline; relate it to validating mirrored route fragments on round-trip booking strings.

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