Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.
Example:
Input: "babad" Output: "bab" Note: "aba" is also a valid answer.
Input: "cbbd" Output: "bb"
和上次做的PAT回文串题目一样，只是返回不是
5. Longest Palindromic Substring
原文：http://www.cnblogs.com/zhangmingzhao/p/7327248.html
