最长公共子序列

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) {
        int len1 = s1.length();
        int len2 = s2.length();
        char[] arr1 = s1.toCharArray();
        char[] arr2 = s2.toCharArray();
        int[][] dp = new int[len1 + 1][len2 + 1]; // s1前i子串与s2前j子串的最长公共子序列长度
        for (int i = 1; i < len1 + 1; i ++) {
            for (int j = 1; j < len2 + 1; 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]);
                }
            }
        }
        // 反推子序列
        StringBuffer sb = new StringBuffer();
        for (int i = len1, j = len2; i >= 0 && j >= 0 && sb.length() < dp[len1][len2]; i --, j --) {
            while (dp[i][j-1] == dp[i][j]) {
                j --;
            }
            while (dp[i-1][j] == dp[i][j]) {
                i --;
            }
            sb.insert(0, arr1[i-1]);
        }
        if (sb.length() == 0) return "-1";
        return sb.toString();
    }
}