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 == null || s1.length() < 1 || s2 == null || s2.length() < 1) {
            return "-1";
        }
        char[] arr1 = s1.toCharArray();
        char[] arr2 = s2.toCharArray();
        int n = arr1.length;
        int m = arr2.length;
        int[][] dp = new int[n + 1][m + 1];
        for (int i = 1; i <= n; i++) {
            for (int j = 1; j <= m; j++) {
                if (arr1[i - 1] == arr2[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]);
                }
            }
        }
        if (dp[n][m] == 0) {
            return "-1";
        }
        char[] res = new char[dp[n][m]];
        int index = res.length - 1;
        int i = n;
        int j = m;
        while (i > 0 && j > 0) {
            if (arr1[i - 1] == arr2[j - 1]) {
                res[index--] = arr1[i - 1];
                i--;
                j--;
            } else {
                if (dp[i - 1][j] > dp[i][j - 1]) {
                    i--;
                } else {
                    j--;
                }
            }
        }
        return new String(res);
    }
}