public void PreorderTraversal()
 {
     if (main != null)
     {
         main.PreOrderTraversal();
     }
 }
            public void PreOrderTraversal()
            {
                Console.Write(data + " ");

                if (leftUnit != null)
                {
                    leftUnit.PreOrderTraversal();
                }

                if (rightUnit != null)
                {
                    rightUnit.PreOrderTraversal();
                }
            }