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18. Valid Palindrome

easyAsked at Reddit

Determine if a string is a palindrome after removing non-alphanumeric characters and lowercasing. Reddit uses this to test two-pointer technique on dirty input — exactly the kind of normalization their username/subreddit-name validation must handle.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Source citations

Public interview reports confirming this problem appears in Reddit loops.

  • Glassdoor (2026-Q1)— Reddit phone screen, used to gauge string-handling carefulness.

Problem

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

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

Example 3

Input
s = " "
Output
true

Approaches

1. Normalize then compare to reverse

Filter to alphanumeric, lowercase, compare to its reverse.

Time
O(n)
Space
O(n)
function isPalindrome(s) {
  const t = s.toLowerCase().replace(/[^a-z0-9]/g, '');
  return t === t.split('').reverse().join('');
}

Tradeoff: Linear but allocates two copies. Acceptable as a first version but not interview-grade.

2. Two pointers, skip non-alnum in-place (optimal)

Pointers from both ends. Skip non-alphanumeric. Compare lowercased characters.

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

Tradeoff: O(1) memory. The skip-in-place is a recurring Reddit pattern for handling dirty user-input (usernames, comment text).

Reddit-specific tips

Reddit interviewers grade on whether you handle the i < j guard inside the inner while loops. Without it, you'll skip past j and crash. Bonus signal: relate this to their input-sanitization for username comparison (case-insensitive, ignore punctuation).

Common mistakes

  • Forgetting i < j in inner skip loops — leads to out-of-bounds.
  • Lowercasing only one side (must compare lowercased to lowercased).
  • Using a regex per character — slow; pre-bind the test function.

Follow-up questions

An interviewer at Reddit may pivot to one of these next:

  • Valid palindrome II (LC 680) — allow one deletion.
  • Longest palindromic substring (LC 5).
  • What if you couldn't use regex?

Solve it now

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Output

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FAQ

Does the empty string count?

Yes — empty reads the same forward and backward. The example with a single space confirms this.

Are accented characters alphanumeric?

Per the spec, only ASCII letters and digits. Constraints guarantee printable ASCII only, so this won't bite.

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