Longest Palindromic Substring

/**
 * @param {string} s
 * @return {string}
 */
var longestPalindrome = function(s) {
  if (!s || s.length === 0)
    return '';
  let start = 0,
    end = 0;
  for (let i = 0; i < s.length; i++) {
    // There can be 2n - 1 centers for the palindromes. We can 
    // include each character and the space in between the       
    // characters.
    let l1 = expandAroundCenters(s, i, i);
    let l2 = expandAroundCenters(s, i, i + 1);
    let l = Math.max(l1, l2);
    if (l > end - start) {
      start = i - Math.floor((l - 1) / 2);
      end = i + Math.floor(l / 2);
    }
  }
  return s.substring(start, end + 1);
};

var expandAroundCenters = function(s, left, right) {
  while (left >= 0 &&
    right < s.length &&
    s.charAt(left) === s.charAt(right)) {
    left--;
    right++;
  }
  return right - left - 1;
}

// Time Complexity O(n ^ 2)
// Space Complexity O(1)
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