//bfs
    public List<Integer> printFromTopToBottom(TreeNode root) {
        List<Integer> ans = new ArrayList<>();
        if(root == null) return ans;
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while(!queue.isEmpty()) {
            int size = queue.size();
            while(size-- > 0) {
                TreeNode node = queue.poll();
                ans.add(node.val);
                if(node.left != null) {
                    queue.offer(node.left);
                }
                if(node.right != null) {
                    queue.offer(node.right);
                }
            }
        }
        return ans;
    }