最长回文字符串

var find = function(start, end, s) {
  while(start >= 0 && end < s.length && s[start] === s[end] && start <= end) {
      start-=1;
      end+=1;
  }
  return [start+1, end-1]; // 因为退出while循环时s[start] ！== s[end]，因此需要回退上一个值
}

var max = function(start1, end1, start2, end2) {
  return (end1 - start1) > (end2 - start2) ? [start1, end1] : [start2, end2];
}

var longestPalindrome = function(s) {
  if(s.length == 1) {
      return s;
  }
  let len = s.length - 1;
  let start = 0, end = 0;
  for(let i= 1;i<=len;i++) {
      const [start1, end1] = find(i,i,s);
      const [start2, end2] = find(i-1, i, s);
      const [maxStart, maxEnd] = max(start1, end1, start2, end2);
      if((maxEnd - maxStart) >= (end - start)) {
          start = maxStart;
          end = maxEnd;
      }
  }
  return s.substring(start, end+1); // substring不包含end下标的值，因此需要+1
  // substr(startPosition, length);
  // substring(startPosition, endPosition + 1)
};