647

class Solution:
    def countSubstrings(self, s: str) -> int:
        n = len(s)
        dp = [[False] * n for _ in range(n)]
        res = 0
        for i in range(n):
            dp[i][i] = True
            res += 1
            if i + 1 < n and s[i] == s[i+1]:
                dp[i][i+1] = True
                res += 1

        for i in range(n-2, -1, -1):
            for j in range(i+1, n):
                if s[i] == s[j] and dp[i+1][j-1]:
                    dp[i][j] = True
                    res += 1

        return res