/*
Change a tree so that the roles of the
left and right pointers are swapped at every node.
So the tree...

        if (node==NULL)  {
             return;
        }
        else { 

             struct node* temp;  

             // do the subtrees
             mirror(node->left);
             mirror(node->right);       

            // swap the pointers in this node
           temp = node->left;
           node->left = node->right;
           node->right = temp;
      }
}