class Solution:
    def countSubstrings(self, s: str) -> int:
        n = len(s)
        ans = 0
        for i in range(2*n-1):
            left = i//2
            right = left + i%2
            while left >= 0 and right < n and s[left] == s[right]:
                left -= 1
                right += 1
                ans += 1
        return ans

字符串 中等

遍历所有回文中心，在中心扩散回文串

https://leetcode-cn.com/problems/palindromic-substrings/