[leedcode 214] Shortest Palindrome

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<s.length();i++){
                seq.insert(0,s.charAt(i));
            }
            return seq.toString();
        }
        return s;
    }
    public boolean isPal(String s){
        int start=0;
        int end=s.length()-1;
        while(start<end){
            if(s.charAt(start)!=s.charAt(end)) return false;
            start++;
            end--;
        }
        return true;
    } 
}