/**
 * @param {string} s
 * @return {string}
 */
var longestPalindrome = function(s) {
  if (s === null || s.length < 1){
    return "";
  }
  
  if (s.length === 1){
    return s;
  }
  
  let start = 0;
  let end = 0;
  
  for (let i = 0; i < s.length; i++){
    let len1 = expandAroundCenter(s, i, i);
    let len2 = expandAroundCenter(s, i, i + 1);
    let len = Math.max(len1, len2);
    if (len > end - start) {
      start = Math.round(i - (len - 1) / 2);
      end = i + len / 2;
    }
  }
  
  return s.substring(start, end+1)
};

function expandAroundCenter(s, left, right) {
  let L = left;
  let R = right;
  
  while (L >= 0 && R < s.length && s.charAt(L) === s.charAt(R)){
    L--;
    R++;
  }
  
  return R - L - 1;
}