代码随想录算法营Day51 ｜ 647. 回文子串，516. 最长回文子序列

647. 回文子串

这题用中心扩散法去计算字符串里有多少回文子串。

class Solution:
    def extend(self,s,i,j):
        res = 0
        while i >= 0 and j<len(s) and s[i] == s[j]:
            res += 1
            i-=1
            j+=1
        return res

    def countSubstrings(self, s: str) -> int:
        res = 0
        for i in range(len(s)):
            res += self.extend(s,i,i)
            res += self.extend(s,i,i+1)
        return res

516. 最长回文子序列

class Solution:
    def longestPalindromeSubseq(self, s: str) -> int:
        n = len(s)
        dp = [[0]*n for _ in range(n)]
        for i in range(n-1,-1,-1):
            dp[i][i] = 1
            for j in range(i+1,n):
                if s[i] == s[j]:
                    dp[i][j] = dp[i+1][j-1] + 2
                else:
                    dp[i][j] = max(dp[i+1][j],dp[i][j-1])
        return dp[0][n-1]