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17. Valid Palindrome

easyAsked at Box

Check whether a string reads the same forward and backward after stripping non-alphanumerics — Box uses this two-pointer pattern in their filename-canonicalization fast paths.

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

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.

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 and reverse

Strip non-alphanumeric, lowercase, compare to its reverse.

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

Walk from both ends, skipping non-alphanumerics; compare lowercased chars.

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:

Box-specific tips

Box wants the no-allocation two-pointer version — they map it to streaming filename-canonicalization checks that run hot in their upload path.

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Output

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