README.md

5. 最长回文子串

English Version

题目描述

给你一个字符串 s，找到 s 中最长的回文子串。

 

示例 1：

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

示例 2：

输入：s = "cbbd"
输出："bb"

示例 3：

输入：s = "a"
输出："a"

示例 4：

输入：s = "ac"
输出："a"

 

提示：

• 1 <= s.length <= 1000
• s 仅由数字和英文字母（大写和/或小写）组成

解法

动态规划法。

设 dp[i][j] 表示字符串 s[i..j] 是否为回文串。

• 当 j - i < 2，即字符串长度为 2 时，只要 s[i] == s[j]，dp[i][j] 就为 true。
• 当 j - i >= 2，dp[i][j] = dp[i + 1][j - 1] && s[i] == s[j]。

Python3

class Solution:
    def longestPalindrome(self, s: str) -> str:
        n = len(s)
        dp = [[False] * n for _ in range(n)]
        start, mx = 0, 1
        for j in range(n):
            for i in range(j + 1):
                if j - i < 2:
                    dp[i][j] = s[i] == s[j]
                else:
                    dp[i][j] = dp[i + 1][j - 1] and s[i] == s[j]
                if dp[i][j] and mx < j - i + 1:
                    start, mx = i, j - i + 1
        return s[start:start + mx]

Java

class Solution {
    public String longestPalindrome(String s) {
        int n = s.length();
        boolean[][] dp = new boolean[n][n];
        int mx = 1, start = 0;
        for (int j = 0; j < n; ++j) {
            for (int i = 0; i <= j; ++i) {
                if (j - i < 2) {
                    dp[i][j] = s.charAt(i) == s.charAt(j);
                } else {
                    dp[i][j] = dp[i + 1][j - 1] && s.charAt(i) == s.charAt(j);
                }
                if (dp[i][j] && mx < j - i + 1) {
                    mx = j - i + 1;
                    start = i;
                }
            }
        }
        return s.substring(start, start + mx);
    }
}

C++

class Solution {
public:
    string longestPalindrome(string s) {
        int n = s.size();
        vector<vector<bool>> dp(n, vector<bool>(n, false));
        int start = 0, mx = 1;
        for (int j = 0; j < n; ++j) {
            for (int i = 0; i <= j; ++i) {
                if (j - i < 2) {
                    dp[i][j] = s[i] == s[j];
                } else {
                    dp[i][j] = dp[i + 1][j - 1] && s[i] == s[j];
                }
                if (dp[i][j] && mx < j - i + 1) {
                    start = i;
                    mx = j - i + 1;
                }
            }
        }
        return s.substr(start, mx);
    }
};

Go

func longestPalindrome(s string) string {
	n := len(s)
	dp := make([][]bool, n)
	for i := 0; i < n; i++ {
		dp[i] = make([]bool, n)
	}
	mx, start := 1, 0
	for j := 0; j < n; j++ {
		for i := 0; i <= j; i++ {
			if j-i < 2 {
				dp[i][j] = s[i] == s[j]
			} else {
				dp[i][j] = dp[i+1][j-1] && s[i] == s[j]
			}
			if dp[i][j] && mx < j-i+1 {
				mx, start = j-i+1, i
			}
		}
	}
	return s[start : start+mx]
}

C#

public class Solution{
    public string LongestPalindrome(string s) {
        int n = s.Length;
        bool[,] dp = new bool[n, n];
        int mx = 1, start = 0;
        for (int j = 0; j < n; ++j)
        {
            for (int i = 0; i <= j; ++i)
            {
                if (j - i < 2)
                {
                    dp[i, j] = s[i] == s[j];
                }
                else
                {
                    dp[i, j] = dp[i + 1, j - 1] && s[i] == s[j];
                }
                if (dp[i, j] && mx < j - i + 1)
                {
                    mx = j - i + 1;
                    start = i;
                }
            }
        }
        return s.Substring(start, mx);
    }
}

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