leetcode 的T5 最长回文字符串

codeloverr2025-03-25 12:20

示例 1：

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

示例 2：

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输入：s = "cbbd"
输出："bb"

提示：

1 <= s.length <= 1000

s 仅由数字和英文字母组成

package edu.contest;
//T5 动态规划
public class Solution4 {

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  int arr[][] = new int[1000][1000];
  int max;
  String maxStr;

  public int calclong(String s, int i, int j) 
  {
  	if (arr[i][j] > 0)
  		return arr[i][j];
  	if (s.charAt(i) == s.charAt(j)) 
  	{
  		if (i == j - 1) 
  		{
  			arr[i][j] = 2;
  			if (arr[i][j] > max) 
  			{
  				max = Math.max(max, j - i + 1);
  				maxStr = (String) s.subSequence(i, j + 1);
  			}
  		}
  		if (i + 1 <= j - 1 && (calclong(s, i + 1, j - 1) != j - 1 - i)) // arr[i+1][j-1]
  		{
  			arr[i][j] = 0;
  		} else if (i + 1 <= j - 1 && calclong(s, i + 1, j - 1) == j - 1 - i) 
  		{
  			arr[i][j] = calclong(s, i + 1, j - 1) + 2;
  			// System.out.println("calclong"+" "+(i+1)+" "+(j-1)+"--->"+
  			if (arr[i][j] > max) 
  			{
  				max = Math.max(max, j - i + 1);
  				maxStr = (String) s.subSequence(i, j + 1);
  			}
  		}
  	} else
  		arr[i][j] = 0;
  	return arr[i][j];
  }

  public String longestPalindrome(String s) {

  	int max = 1;
  	// System.out.println(calclong(s,1,2));
  	
  	for (int i = 0; i < s.length(); i++)
  		arr[i][i] = 1;
  	maxStr = "" + s.charAt(0);
  	for (int i = 0; i < s.length(); i++)
  		for (int j = i + 1; j < s.length(); j++) {
  			arr[i][j] = calclong(s, i, j);
  		}
  	/*
  	 * for(int i=0;i<s.length();i++) { for(int j=0; j<s.length(); j++)
  	 * System.out.print(arr[i][j]+"\t"); System.out.println(); }
  	 * System.out.println(maxStr);
  	 */
  	return maxStr;
  }

  public static void main(String[] args) {
  	new Solution4().longestPalindrome("aaaa");
  }

}