Given a string s, count how many of its substrings are palindromes. Substrings at different positions count separately even if their text is identical; every single character counts as one.
Examples
Input: s = "abba"
Output: 6
Explanation: "a", "b", "b", "a", "bb", "abba".
Input: s = "abc"
Output: 3
Constraints
0 <= len(s) <= 2000- Target: O(n^2) time, O(1) extra space.
Goals
- Count palindromic substrings by position, not by distinct content
- Reuse the expand-around-centre idea to count instead of to find
- Verify counts on tiny inputs by hand