题解 | #最长公共子序列(二)#

import java.util.*;


public class Solution {
    /**
     * longest common subsequence
     * @param s1 string字符串 the string
     * @param s2 string字符串 the string
     * @return string字符串
     */
    public String LCS (String s1, String s2) {
        // write code here
        if(s1.equals("") || s2.equals("")) return "-1";

        int m = s1.length();
        int n = s2.length();


        int dp[][] =new int[m+1][n+1];

        for(int i = 1 ; i <= m ; i++){
            for(int j = 1 ; j <= n ; j++){
                if(s1.charAt(i-1) == s2.charAt(j-1)){
                    dp[i][j] = dp[i-1][j-1] + 1;
                }else{
                    dp[i][j] = Math.max(dp[i-1][j],dp[i][j-1]);
                }
            }
        }

        StringBuilder s = new StringBuilder("");

        while(m != 0  || n != 0 ){
                while( m > 0 && dp[m][n] == dp[m-1][n] ) m--;
                while(n > 0 && dp[m][n] == dp[m][n-1] ) n--;

                if(m>0){
                    s.append(s1.charAt(m-1));
                    m--;
                    n--;
                }
            }
        s = s.reverse();
        String res = s.toString();
        return res.equals("")?"-1":res;

    }


}