题目描述

从上往下打印出二叉树的每个节点，同层节点从左至右打印。

分析

用arraylist模拟一个队列来存储相应的TreeNode

贴出代码

import java.util.ArrayList;/**public class TreeNode {    int val = 0;    TreeNode left = null;    TreeNode right = null;    public TreeNode(int val) {        this.val = val;    }}*/public class Solution {    public ArrayList
    
      PrintFromTopToBottom(TreeNode root) {        ArrayList
     
       list = new ArrayList<>();        ArrayList
      
        queue = new ArrayList<>();        if(root == null) {            return list;        }        queue.add(root);        while(queue.size() != 0) {            TreeNode temp = queue.remove(0);            if(temp.left != null) {                queue.add(temp.left);            }            if(temp.right != null) {                queue.add(temp.right);            }            list.add(temp.val);        }        return list;    }}