Leetcode 刷题笔记1 动态规划part13

leetcode  647 回文子串

暴力解法：

class Solution:
    def countSubstrings(self, s: str) -> int:
        ans = 0
        for i in range(1, len(s) + 1):
            for j in range(len(s) - i + 1):
                if s[j:j + i] == s[j:j + i][::-1]:
                    ans +=1
        return ans

dp：

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

中心扩散法（双指针法）：

class Solution:
    def countSubstrings(self, s: str) -> int:
        ans = 0
        for i in range(len(s)):
            ans += self.extend(s, i, i,len(s))
            ans += self.extend(s, i, i+1,len(s))
        return ans
    def extend(self, s, i, j, n):
        ans = 0
        while i >= 0 and j < n and s[i] == s[j]:
            i -= 1
            j += 1
            ans += 1
        return ans

leetcode 516 最长回文子串

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