void EnterNode(TreePointer *node, int num)
{/* Enter an element into the tree */
/* pass in the address of the pointer to the current node */
  TreePointer ptr;
  ptr = *node;
  if (!ptr)  {
     ptr = (TreePointer)malloc(sizeof(struct TreeNode));
     ptr->data = num;
     ptr->LeftChild =  ptr->RightChild = NULL;
     *node = ptr;
  }
  else if (num <= ptr->data)
     EnterNode(&ptr->LeftChild, num);
  else
     EnterNode(&ptr->RightChild, num);
}