从上往下打印二叉树

 1 import java.util.*;
 2 /**
 3 public class TreeNode {
 4     int val = 0;
 5     TreeNode left = null;
 6     TreeNode right = null;
 7 
 8     public TreeNode(int val) {
 9         this.val = val;
10 
11     }
12 
13 }
14 */
15 public class Solution {
16     
17     Queue<TreeNode> queue = new LinkedList<>();
18     ArrayList<Integer> arr = new ArrayList();
19     
20     public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
21         
22         if(root==null)
23             return arr;
24         queue.add(root);
25         while(!queue.isEmpty()){
26             TreeNode t = queue.poll();//好久没用过队列函数不太清楚了！！！
27             arr.add(t.val);
28             if(t.left!=null)
29                 queue.add(t.left);
30             if(t.right!=null)
31                 queue.add(t.right);
32         }
33         return arr;
34     }
35 }