【LeetCode】【5】最长回文子串

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系列专栏：LeetCode

刷题指南：LeetCode刷题指南

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题目描述

• 给一个字符串s，找到s中最长的回文子串

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样例输入输出与解释

样例1

• 输入：s = "babad"
• 输出："bab"
• 解释："aba"同样是符合题意的答案

样例2

• 输入：s = "cbbd"
• 输出："bb"

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提示

• 1 <= s.length <= 1000
• s仅由数字和英文字母组成

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Python实现

动态规划

class Solution:
    def longestPalindrome(self, s: str) -> str:
        n = len(s)

        if n < 2:
            return s

        max_len = 1
        begin = 0

        dp = [[False] * n for _ in range(n)]
        for i in range(n):
            dp[i][i] = True

        for l in range(2, n + 1):
            for i in range(n - l + 1):
                j = l + i - 1

                if s[i] != s[j]:
                    dp[i][j] = False
                else:
                    if j - i < 3:
                        dp[i][j] = True
                    else:
                        dp[i][j] = dp[i + 1][j - 1]

                if dp[i][j] and j - i + 1 > max_len:
                    max_len = j - i + 1
                    begin = i

        return s[begin:begin + max_len]

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