面试题：从上往下打印二叉树

import java.util.ArrayList;
import java.util.Queue;
import java.util.LinkedList;
public class Solution {
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
        ArrayList<Integer> list=new ArrayList<>();
        if(root==null) return list;
        Queue<TreeNode> queue=new LinkedList<TreeNode>();
        queue.add(root);
        while(!queue.isEmpty()){
            TreeNode node=queue.poll();
            list.add(node.val);
            if(node.left!=null)
                queue.add(node.left);
            if(node.right!=null)
                queue.add(node.right);
        }
        return list;
    }
}