从上往下打印二叉树

 public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
        ArrayList<Integer> arrayList=new ArrayList<>();
        if (root==null){
            return arrayList;
        }
        Queue<TreeNode> queue=new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()){
            root=queue.poll();
            arrayList.add(root.val);
            if (root.left!=null){
                queue.offer(root.left);
            }
            if (root.right!=null){
                queue.offer(root.right);
            }
        }
        return arrayList;
    }