【代码随想录训练营】【Day 60】【动态规划-13】| Leetcode 647, 516

花花啊啦啦啦2024-06-30 9:31

需强化知识点

动态规划-回文系列

题目

647. 回文子串

注意sum（dp）不行
j的遍历要倒序，并且 for j in range(i, n):

python 复制代码

class Solution:
    def countSubstrings(self, s: str) -> int:
        n = len(s)
        dp = [[0]*(n) for _ in range(n)]
        result = 0

        for i in range(n-1, -1, -1):
            for j in range(i, n):
                if s[i] == s[j]:
                    if j - i == 0:
                        dp[i][j] = 1
                        result += 1
                    elif abs(j - i) ==  1:
                        dp[i][j] = 1
                        result += 1
                    else:
                        dp[i][j] = dp[i+1][j-1]
                        if dp[i+1][j-1]:
                            result += 1
                else:
                    dp[i][j] = 0
        
        return sum([sum(sublist) for sublist in dp])

516. 最长回文子序列

注意 python int max为 2**31 -1
滑动窗口，固定结束位置的方式，然后注意nums包含0，所以while判断是>=

python 复制代码

class Solution:
    def longestPalindromeSubseq(self, s: str) -> int:
        n = len(s)
        # 以 i，j 的 最长序列长度
        dp  = [[0] * (n) for _ in range(n)]
        for i in range(n):
            dp[i][i] = 1

        for i in range(n-1, -1, -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]