https://leetcode.com/problems/longest-palindromic-substring/#/description
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 - Unsolved
原文：http://www.cnblogs.com/prmlab/p/7231563.html