private void rec_traverse(ref Node tree)
        {
            if (tree != null)
            {
                if (tree.left == null)
                {
                    Console.WriteLine(tree.info);
                }
                else
                {
                    rec_traverse(ref tree.left);
                }
 
                if (tree.middle == null)
                {
                    Console.WriteLine(tree.info);
                }
                else
                {
                    rec_traverse(ref tree.middle);
                }
 
                if (tree.right == null)
                {
                    Console.WriteLine(tree.info);
                }
                else
                {
                    rec_traverse(ref tree.right);
                }
            }
        }