Given a string S, you are allowed to convert it to a palindrome by adding characters in front of it. Find and return the shortest palindrome you can find by performing this transformation.

For example:

Given "aacecaaa", return "aaacecaaa".

Given "abcd", return "dcbabcd".

public class Solution {    public String shortestPalindrome(String s) {/*        其实问题可以转化成求从s[0]开始的最长回文，找到以s[0]开始的最长回文，将剩下的部分倒序补在字符串前面就是答案        可以从最后一个往前找        先找到的就是以s[0]为开始的最长回文        此种方法会超时        TLE        http://blog.csdn.net/yujin753/article/details/47047155        */        if(s==null||s.length()<=0) return null;        int len=s.length();        for(;len>0;len--){            if(isPal(s.substring(0,len)))               break;        }        if(len!=s.length()){            StringBuilder seq=new StringBuilder(s.substring(0,len));            for(int i=len;i