5. Longest Palindromic Substring
[https://leetcode.com/problems/longest-palindromic-substring/]
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"
public class