1.说明：
最长回文串，考虑到长度为奇数的回文串和长度为偶数的回文串。
2.代码：
var longestPalindrome = function(s) {var max = 0;var mark = {i: 0,j: 0};var result = '';var getOddLength = function (s, index) {var i = index;var j = index;while (i >= 0 && j < s.length) {if (s[i] === s[j]) {i--;j++;}else {break;}}return j - i + 1;};var getEvenLength = function (s, index) {if (index >= s.length ) {return 0;}var i = index;var j = index + 1;while (i >= 0 && j < s.length) {if (s[i] === s[j]) {i--;j++;}else {break;}}return j - i + 1;};var getResult = function (mark, s) {var i = mark.i;var j = mark.j;var str = '';while (i >= 0 && j < s.length) {if (s[i] === s[j]) {str = s[i] + str;if (i !== j) {str = str + s[j];}i--;j++;}else {break;}}return str;};for (var i = 0; i < s.length; i++) {var l1 = getOddLength(s, i);var l2 = getEvenLength(s, i);if (l1 > max) {max = l1;mark.i = i;mark.j = i;}if (l2 > max) {max = l2;mark.i = i;mark.j = i + 1;}}return getResult(mark, s);
};