function longestPalindrome(s) {
  if (s.length < 2) return s;
  let start = 0, end = 0;

  const expand = (left, right) => {
    while (left >= 0 && right < s.length && s[left] === s[right]) {
      left--;
      right++;
    }
    return [left + 1, right - 1];
  };

  for (let i = 0; i < s.length; i++) {
    const [l1, r1] = expand(i, i);
    if (r1 - l1 > end - start) {
      start = l1; end = r1;
    }
    const [l2, r2] = expand(i, i + 1);
    if (r2 - l2 > end - start) {
      start = l2; end = r2;
    }
  }

  return s.slice(start, end + 1);
}