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".

分析：

就是找出从原字符串中从第一个字符开始能够组成的最长palindome. 然后把原字符串中后面部分reverse后放在前面，这样整个字符串就是palindrome, 并且长度最小。

 public class Solution {
     public String shortestPalindrome(String s) {
         if (s == null || s.length() <= )
             return s;
         int size = ;
         for (int i = ; i <= (s.length() - ) / ; i++) {
             size = Math.max(size, maxSize(s, i, i));
             size = Math.max(size, maxSize(s, i, i + ));
         }
         String str = s.substring(size);
         StringBuilder sb = new StringBuilder(str);
         sb.reverse();
         return sb.toString() + s;
     }

     private int maxSize(String s, int left, int right) {
         while (left >=  && right < s.length() && s.charAt(left) == s.charAt(right)) {
             left--;
             right++;
         }
         if (left == -) {
             return right - left - ;
         }
         return ;
     }
 }