最大回文子串

中心扩展法.js

/**
 * @param {string} s
 * @return {string}
 */
var longestPalindrome = function(s) {	
  if (!s) return '';
  var longest = s[0];
  var expandAroundCenter = function (left, right) {
    while (left >= 0 && right < s.length && s[left] === s[right]) {
      left--;
      right++;
    }
    return s.slice(left + 1, right);
  }
  for (var i = 0; i < s.length; i++) {
    // 奇数
    var odd = expandAroundCenter(i, i);
    if (odd.length > longest.length) longest = odd;
    // 偶数
    var even = expandAroundCenter(i, i + 1);
    if (longest.length < even.length) longest = even;
  }
  return longest;
};