Given a string s, find the longest palindromic substring（回文子串） in s. You may assume that the maximum length of s is 1000.
Example 1:
Input: "babad"
Output: "bab"
Note: "aba" is also a valid answer.
Example 2:
Input: "cbbd"
Output: "bb"
https://leetcode.com/problems/longest-palindromic-substring/