static void InfixPrintln(TreeNode<int> tree, bool f = true)
        {
            if (tree == null)
            {
                Console.WriteLine("<empty line>");
                return;
            }
            if (tree.left != null)
                InfixPrintln(tree.left, false);
            Console.Write(tree.data + " ");
            if (tree.right != null)
                InfixPrintln(tree.right, f);
            else if (f)
                Console.WriteLine();
        }