5\. Longest Palindromic Substring

Medium

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.