自行摸索的版本，部分代码可能有冗余

import java.io.*;
import java.util.*；

public class 二叉树遍历 {
    static char[] inorder, layer;
    static boolean[] flag;
    static BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
    static BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(System.out));
    public static void main(String[] args) throws IOException {
        char[] temp = reader.readLine().trim().toCharArray();
        ArrayDeque<Node> deque = new ArrayDeque<>(temp.length);
        inorder = new char[temp.length+2];//输入的中序遍历顺序
        layer = new char[temp.length+2];//输入的层序遍历顺序
        //用于保存中序遍历顺序下某个值是否被遍历到，多申请两个是为了方便处理边界
        flag = new boolean[temp.length+2];
        //用于保存某个元素在中序遍历顺序中的位置
        HashMap<Character, Integer> map = new HashMap<>(temp.length);
        //读入数据
        for (int i = 1; i <= temp.length; i++) {
            inorder[i] = temp[i-1];
            map.put(inorder[i],i);
        }
        temp = reader.readLine().trim().toCharArray();
        for (int i = 1; i <= temp.length; i++) {
            layer[i] = temp[i-1];
        }

        //初始化头结点并将其加入队列
        Node head = new Node(layer[1]);
        deque.add(head);
        //此处将第一个与最后一个赋值为true用于确定边界，避免每次循环时重复判定是否越过了边界
        flag[0] = flag[flag.length-1] = true;
        flag[map.get(head.value)] = true;//将头结点变更为“已访问”
        int index = 1;//用于保存当前访问到层序遍历中的哪一个值
        while(!deque.isEmpty()){
            Node node = deque.poll();
            //如果当前节点在先序遍历的顺序里的左侧的值未被访问过，则说明当前节点有左孩子
            if (!flag[map.get(node.value)-1]){
                node.left = new Node(inorder[map.get(layer[++index])]);
                deque.addLast(node.left);
                flag[map.get(node.left.value)] = true;
            }
            //如果当前节点在先序遍历的顺序里的右侧的值未被访问过，则说明当前节点有右孩子
            if (!flag[map.get(node.value)+1]){
                node.right = new Node(inorder[map.get(layer[++index])]);
                deque.addLast(node.right);
                flag[map.get(node.right.value)] = true;
            }
        }
        out(head);//用于递归输出先序遍历的结果
        writer.flush();
    }
    static class Node{
        char value;
        Node left;
        Node right;
        public Node(char value) {
            this.value = value;
            left = null;
            right = null;
        }
    }
    public static void out(Node node) throws IOException {
        writer.write(node.value+"");
        if (node.left != null)
            out(node.left);
        if (node.right != null)
            out(node.right);
    }
}