按之字形顺序打印二叉树

import java.util.ArrayList;
import java.util.Stack;

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

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

    }

}
*/
public class Solution {
    public ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {//大体思路 ：因为按层进行遍历，所以使用广搜；因为两层之间的存放顺序是相反的，可以利用栈的先进后出
      ArrayList<ArrayList<Integer>>lists=new ArrayList<ArrayList<Integer>>();
      Stack<TreeNode>stack1=new Stack<TreeNode>();//用来存放奇数层结点
      Stack<TreeNode>stack2=new Stack<TreeNode>();//用来存放偶数层结点
        stack1.push(pRoot);
       if(pRoot==null)
           return lists;
        int ans=1;
        while(stack1.empty()==false||stack2.empty()==false){//进行广搜
            if(ans==1){//奇数层时
                ArrayList<Integer>list=new ArrayList<Integer>();
                while(stack1.empty()==false){
                    TreeNode tree=stack1.peek();
                    list.add(tree.val);
                    stack1.pop();
                    if(tree.left!=null)//按照 先左后右的顺序
                    stack2.push(tree.left);
                    if(tree.right!=null)
                    stack2.push(tree.right);
                }
                lists.add(list);
            }
            else{//偶数层
                ArrayList<Integer>list=new ArrayList<Integer>();
                while(stack2.empty()==false){
                    TreeNode tree=stack2.peek();
                    list.add(tree.val);
                    stack2.pop();
                    if(tree.right!=null)// 按照先右后左的顺序
                    stack1.push(tree.right);
                    if(tree.left!=null)
                    stack1.push(tree.left);
                }
                lists.add(list);
            }
            ans^=1;//标记层数
        }
        return lists;
    }

}