647. Palindromic Substrings

Medium

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.

Solution

class Solution:
    def countSubstrings(self, s: str) -> int:
        def expand(l: int, r: int) -> int:
            count = 0

            while l >= 0 and r < len(s) and s[l] == s[r]:
                count += 1
                l -= 1
                r += 1

            return count

        res = 0

        for i in range(len(s)):
            res += expand(i, i)
            res += expand(i, i + 1)

        return res