请实现一个函数按照之字形打印二叉树，即第一行按照从左到右的顺序打印，第二层按照从右至左的顺序打印，第三行按照从左到右的顺序打印，其他行以此类推。

import java.util.ArrayList;

/*
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;

    }

}
*/
import java.util.*;
public class Solution {
    public ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {
        ArrayList<ArrayList<Integer>> arrayList = new ArrayList<ArrayList<Integer>>();
        if( pRoot == null ){
            return arrayList;
        }
        
        Stack<TreeNode> s1 = new Stack<TreeNode>();
        s1.add( pRoot );
        Stack<TreeNode> s2 = new Stack<TreeNode>();
        while( !s1.empty() || !s2.empty()){
            if(!s1.empty()){
                ArrayList<Integer> list = new ArrayList<Integer>();
                while(!s1.empty()){
                    TreeNode node = s1.pop();
                    list.add(node.val);
                    if(node.left != null){
                        s2.add(node.left);
                    }
                    if(node.right != null){
                        s2.add(node.right);
                    }
                }
                arrayList.add(list);
            }else{
                ArrayList<Integer> list = new ArrayList<Integer>();
                while(!s2.empty()){
                
                    TreeNode node = s2.pop();
                    list.add(node.val);
                
                    if(node.right != null){
                        s1.add(node.right);
                    }
                    if(node.left != null){
                        s1.add(node.left);
                    }
                }
                arrayList.add(list);
            }
        }
       return arrayList;
        
    }

}