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.Example:
Input: "cbbd" Output: "bb"
5. Longest Palindromic Substring
原文：http://www.cnblogs.com/naivecoder/p/7039646.html