9. Valid Palindrome

easyAsked at ByteDance

Determine if a string is a palindrome ignoring non-alphanumeric characters — ByteDance uses it as a moderation-pipeline warm-up before harder text-normalization questions.

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

Problem

Given a string s, return true if it reads the same forward and backward after lowercasing and removing all non-alphanumeric characters. Otherwise return false.

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. Filter and reverse

Strip non-alphanumerics, lowercase, then compare with reversed copy.

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

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

Tradeoff:

2. Two pointers in-place

Walk pointers in from both ends skipping non-alphanumerics; bail on the first mismatch.

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) {
    if (!ok(s[i])) { i++; continue; }
    if (!ok(s[j])) { j--; continue; }
    if (s[i].toLowerCase() !== s[j].toLowerCase()) return false;
    i++; j--;
  }
  return true;
}

Tradeoff:

ByteDance-specific tips

ByteDance values candidates who normalize text exactly once and connect it to how their content-moderation pipelines pre-tokenize comments at scale.

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Output

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