Given a string s, return the longest palindromic substring in s.

Example 1:

Input: s = "babad" Output: "bab" Note: "aba" is also a valid answer. Example 2:

Input: s = "cbbd" Output: "bb" Example 3:

Input: s = "a" Output: "a" Example 4:

Input: s = "ac" Output: "a"

Constraints:

1 <= s.length <= 1000 s consist of only digits and English letters.