leetcode5最长回文串

Posted 2022-02-23 lisin-lee-cooper

一.问题描述

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。

二.示例代码

public class LongestPalindromeString 

    public static void main(String[] args) 
        String palindrome = "babad";
        String result = palindromeString2(palindrome);
        System.out.println(result);
    

    private static String palindromeString(String palindrome) 

        int len = palindrome.length();
        if (len < 2) 
            return palindrome;
        
        boolean[][] dp = new boolean[len][len];
        char[] chars = palindrome.toCharArray();
        int begin = 0, maxLen = 1;
        for (int i = 0; i < len; i++) 
            dp[i][i] = true;
        

        for (int i = 0; i < len; i++) 
            for (int j = i + 1; j < len; j++) 
                if (chars[i] != chars[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] && j - i + 1 > maxLen) 
                    maxLen = j - i + 1;
                    begin = i;
                
            
        

        return palindrome.substring(begin, begin + maxLen);

    

    private static String palindromeString2(String palindrome) 
        if (palindrome.length() < 2) 
            return palindrome;
        
        int begin = 0, maxLen = 1;
        char[] chars = palindrome.toCharArray();
        for (int i = 0; i < palindrome.length() - 1; i++) 
            for (int j = i + 1; j < palindrome.length(); j++) 
                if (j - i + 1 > maxLen && validPalindrome(chars, i, j)) 
                    maxLen = j - i + 1;
                    begin = i;
                
            
        
        return palindrome.substring(begin, begin + maxLen);
    

    private static boolean validPalindrome(char[] chars, int i, int j) 
        while (i < j) 
            if (chars[i++] != chars[j--]) 
                return false;
            
        

        return true;