题目描述：

思路：

两种方法：

①使用标记偶数行，如果是偶数行使用reverse把列表反转。

②使用两个栈实现。

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>> result = new ArrayList<ArrayList<Integer>>();
        if(null==pRoot)
            return result;
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.offer(pRoot);
        ArrayList<Integer> arr = new ArrayList<Integer>();
        boolean isoushu = false;
        while(!queue.isEmpty()){
            int size = queue.size();
            for(int i=0;i<size;i++){
                TreeNode temp = queue.poll();
                arr.add(temp.val);
                if(temp.left!=null)
                    queue.offer(temp.left);
                if(temp.right!=null)
                    queue.offer(temp.right);
            }
            if(isoushu){
              Collections.reverse(arr);   
              isoushu=!isoushu;
            }
            result.add(arr);
            arr= new ArrayList<Integer>();
        }
        return result;    
    }

}

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) {
        int layer = 1;
        //s1存奇数层节点
        Stack<TreeNode> s1 = new Stack<TreeNode>();
        s1.push(pRoot);
        //s2存偶数层节点
        Stack<TreeNode> s2 = new Stack<TreeNode>();
         
        ArrayList<ArrayList<Integer>> list = new ArrayList<ArrayList<Integer>>();
         
        while (!s1.empty() || !s2.empty()) {
            if (layer%2 != 0) {
                ArrayList<Integer> temp = new ArrayList<Integer>();
                while (!s1.empty()) {
                    TreeNode node = s1.pop();
                    if(node != null) {
                        temp.add(node.val);
                        System.out.print(node.val + " ");
                        s2.push(node.left);
                        s2.push(node.right);
                    }
                }
                if (!temp.isEmpty()) {
                    list.add(temp);
                    layer++;
                    System.out.println();
                }
            } else {
                ArrayList<Integer> temp = new ArrayList<Integer>();
                while (!s2.empty()) {
                    TreeNode node = s2.pop();
                    if(node != null) {
                        temp.add(node.val);
                        System.out.print(node.val + " ");
                        s1.push(node.right);
                        s1.push(node.left);
                    }
                }
                if (!temp.isEmpty()) {
                    list.add(temp);
                    layer++;
                    System.out.println();
                }
            }
        }
        return list;
    }

}