Given a string `s`, return the number of palindromic substrings in it.

A string is a palindrome when it reads the same backward as forward.

A substring is a contiguous sequence of characters within the string.

Example 1:

    Input: s = "abc"
    Output: 3
    Explanation: Three palindromic strings: "a", "b", "c".

Example 2:

    Input: s = "aaa"
    Output: 6
    Explanation: Six palindromic strings: "a", "a", "a", "aa", "aa", "aaa".

Constraints:

- `1 <= s.length <= 1000`
- `s` consists of lowercase English letters.