从上往下打印出二叉树的每个节点，同层节点从左至右打印。

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.Queue;
//即广度优先搜索树
public class Solution {
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
        
        ArrayList<Integer> list = new ArrayList<Integer>();

        if(root == null){
            return list;
        }

        Queue<TreeNode> q = new LinkedList<TreeNode>();
        q.offer(root);

        while (!q.isEmpty()){
            TreeNode tree  = q.poll();
            if(tree.left!=null){
                q.offer(tree.left);
            }
            if(tree.right!=null){
                q.offer(tree.right);
            }
            list.add(tree.val);
        }

        return list;
    }
}