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;
/** public class TreeNode { int val = 0; TreeNode left = null; TreeNode right = null; public TreeNode(int val) { this.val = val; } } */
import java.util.Deque;
import java.util.LinkedList;
import java.util.ArrayList;
 
public class Solution {
   
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
   
        ArrayList<Integer> list=new ArrayList<>();
        if(root==null)
            return list;
        Deque<TreeNode> q=new LinkedList<TreeNode>();
        q.add(root);
        while(!q.isEmpty()){
   
            TreeNode t=q.pop();
            list.add(t.val);
            if(t.left!=null){
   
                q.add(t.left);
            }
            if(t.right!=null){
   
                q.add(t.right);
            }
        }
        return list;
    }
}