题解 | #最长公共子串#

用动态规划法解题。

import java.util.*;


public class Solution {
    /**
     * longest common substring
     * @param str1 string字符串 the string
     * @param str2 string字符串 the string
     * @return string字符串
     */
    public String LCS (String str1, String str2) {
        // write code here
        if (str1 == null || str2 == null || str1.equals(" ") || str2.equals(" ")) {
            return "-1";
        }
        //动态规划  二维数组，斜下来就是最长字串
        int str1len = str1.length();
        int str2len = str2.length();
        int maxLen = 0;
        int index = 0;
//        定义一个二维数组
        int[][] dp = new int[str1len][str2len];

        //第一步：划分
        for (int i = 0; i < str1len; i++) {
            for (int j = 0; j < str2len; j++) {
                if (str1.charAt(i) == str2.charAt(j)) {
                    //防止数组越界
                    if (j * i ==0){
                        dp[i][j]=1;
                        //if条件语句保证maxLen不被破坏
                        if(maxLen < dp[i][j]){
                            maxLen = dp[i][j];
                            index = i;
                        }
                    } else{
                        dp[i][j] = dp[i - 1][j - 1] + 1;
                        //if条件语句用来更新maxLen
                        if(maxLen < dp[i][j]){
                            maxLen = dp[i][j];
                            index = i;

                        }
                    }
                }
            }
        }
        if( maxLen==0 ){
            return "-1";
        }
        return str1.substring(index-maxLen+1,index+1);
    }
}