005.longest-palindromic-substring

5. 最长回文子串

给你一个字符串 s，找到 s 中最长的回文子串。

如果字符串的反序与原始字符串相同，则该字符串称为回文字符串。

 

示例 1：

输入：s = "babad"
输出："bab"
解释："aba" 同样是符合题意的答案。

示例 2：

输入：s = "cbbd"
输出："bb"

 

提示：

• 1 <= s.length <= 1000
• s 仅由数字和英文字母组成

Hints

1. How can we reuse a previously computed palindrome to compute a larger palindrome?
2. If “aba” is a palindrome, is “xabax” a palindrome? Similarly is “xabay” a palindrome?
3. Complexity based hint:
   If we use brute-force and check whether for every start and end position a substring is a palindrome we have O(n^2) start - end pairs and O(n) palindromic checks. Can we reduce the time for palindromic checks to O(1) by reusing some previous computation.