Fresher · Two pointers

Palindrome · Compare from Both Ends

For an ASCII string, ignore non-alphanumeric characters and case. Return whether the remaining characters read the same in both directions. An empty normalized string is a palindrome.

Examples

"A man, a plan, a canal: Panama" → true

Compare approaches

Normalize and reverse

Construct a normalized string and compare it with its reversal.

Time: O(n) · Space: O(n)

function palindrome(text) { const s = text.toLowerCase().replace(/[^a-z0-9]/g, ""); return s === [...s].reverse().join(""); }

Two pointers

Skip punctuation in place and compare only the next valid pair. The ASCII contract avoids Unicode normalization ambiguity.

Time: O(n) · Space: O(1)

function palindrome(text) {
  let left = 0, right = text.length - 1;
  const valid = c => /[a-z0-9]/i.test(c);
  while (left < right) {
    if (!valid(text[left])) { left++; continue; }
    if (!valid(text[right])) { right--; continue; }
    if (text[left].toLowerCase() !== text[right].toLowerCase()) return false;
    left++; right--;
  }
  return true;
}

Common traps

Do not compare spaces or punctuation.
Define character and case rules before coding.

Practise in the workspace →