两年后的相遇……
题主现在应该已经是大神了吧……

你要的java，但我觉得可能你已经用不上了

import java.io.File;
import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Scanner;

public class Main {
    static FileReader fr;
    static char[] mQueue;
    static char[] eQueue;

    public static Scanner getInput() {
        return new Scanner(System.in);
    }

//  public static Scanner getInput() throws IOException {
//      File f = new File("input.txt");
//      fr = new FileReader(f);
//      Scanner sc = new Scanner(fr);
//
//      return sc;
//  }
    
    public static void input() throws IOException {
        Scanner sc = getInput();
        String s1, s2;
        s1 = sc.nextLine();
        s2 = sc.nextLine();
        
        mQueue = s1.toCharArray();
        eQueue = s2.toCharArray();
        if (fr != null) {
            fr.close();
        }
    }
    
    static class Node{
        char key;
        public Node left;
        public Node right;
        
        public Node(char key) {
            this.key = key;
        }
    }
    
    public static Node check(int mb, int me, int eb, int ee)
    {
        if (mb > me || eb > ee) {
            return null;
        }
        Node node = new Node(eQueue[ee]);
        
        for (int len = 0; len <= me - mb; len ++) {
            int index = len + mb;
            if (mQueue[index] == node.key) {
                node.left = check(mb, index - 1, eb, eb + len - 1);
                node.right = check(index + 1, me, len + eb, ee - 1);
                break;
            }
        }
        return node;
    }
    
    static String encode(Node root) {
        if (root == null) {
            return "";
        }
        String s = new String(String.valueOf(root.key));
        s = s + encode(root.left) + encode(root.right);
        return s;
    }
    
    public static String algorithm()
    {
        Node root = check(0, mQueue.length - 1, 0, eQueue.length - 1);
        String result = encode(root);
        return result;
    }

    public static void output(String result) {
        System.out.println(result);
    }

    public static void main(String args[]) throws IOException {
        input();
        String result = algorithm();
        output(result);
    }
}