import java.util.ArrayList;

 /**

 public class TreeNode {

     int val = 0;

     TreeNode left = null;

     TreeNode right = null;

     public TreeNode(int val) {

         this.val = val;

     }

 }

 */

 //用ArrayList来模拟一个队列

 public class Solution {

     public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {

         ArrayList<Integer> list = new ArrayList<>();

         ArrayList<TreeNode> 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;

     }

 }