按之字形顺序打印二叉树

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


public class Solution {
    public ArrayList > Print(TreeNode pRoot) {

        //数的层数
        int layer = 1;
        //存放奇数层的TreeNode
        Stack s1 = new Stack();
        //存放偶数层的TreeNode
        Stack s2 = new Stack();
        
        ArrayList> resultList = new ArrayList>();
        
        //用于存放每一层的结点的值
        ArrayList tempList;
        
        s1.push(pRoot);
        
        while(!s1.empty() || !s2.empty()){
            
            tempList = new ArrayList<>();
            
            if(layer%2 != 0){
                
                
                while(!s1.empty()){
                    //取出第一个TreeNode
                    TreeNode node = s1.pop();
                    if(node != null){
                        tempList.add(node.val);
                        //因为下一层是偶数层，是从右向左遍历，而Stack是先进后出，所以先node.left，后node.right
                        s2.push(node.left);
                        s2.push(node.right);
                    }
                }
                
            }else{
                
                
                while(!s2.empty()){
                    //取出第一个TreeNode
                    TreeNode node = s2.pop();
                    if(node != null){
                        tempList.add(node.val);
                        //因为下一层是奇数层，是从左向右遍历，而Stack是先进后出，所以先node.right，后node.left
                        s1.push(node.right);
                        s1.push(node.left);
                    }  
                }
            }
            
            if(!tempList.isEmpty()){
                resultList.add(tempList);
            }
            
            //层数加一
            layer++;
        }
        
        return resultList;
    }

}