import java.util.ArrayList;

import java.util.LinkedList;

/**

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) {

        //定义一个辅助队列

        LinkedList<TreeNode> queue=new LinkedList<>();

        //存储一层的打印结果

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

        if(root==null){

            return resultCur;

        }

        //定义两个变量last和nlast，分别记录当前行的最右节点和下一行的最右节点

        TreeNode last=root;

        TreeNode nlast=root;

        TreeNode cur=root;

        queue.add(root);

        while(queue.size()!=0){

            cur=queue.poll();

            resultCur.add(cur.val);

            if(cur.left!=null){

                queue.add(cur.left);

                nlast=cur.left;

            }

            if(cur.right!=null){

                queue.add(cur.right);

                nlast=cur.right;

            }

            if(cur==last){

                last=nlast;

            }

        }

        return resultCur;

    }

}