从上往下打印二叉树(java)

import java.util.ArrayList;
import java.util.*;
/**
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;

    }

}
*/
public class Solution {
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
        ArrayList<Integer> list=new ArrayList<Integer>();
        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;
    }
}