5. Longest Palindromic Substring
Given a string s, return the longest palindromic substring in s.
Example 1:
Input: s = "babad"
Output: "bab"
Explanation: "aba" is also a valid answer.
Example 2:
Input: s = "cbbd"
Output: "bb"
有三种解法：
- brute force: time complexity: O(n^3)
- dp
- 中心扩散法
