void SetSize(TreeNode * t)
// precondition:  t != NULL
// postcondition: The size field of every node in the tree t
//                has been set to the size of that node's subtree    
{
    int leftSize  = 0;   // size of left subtree
    int rightSize = 0;   // size of right subtree
    
    if (t != NULL)
    {
	SetSize(t->left);
	SetSize(t->right);

	if (t->left != NULL)  leftSize  = t->left->size;
	if (t->right != NULL) rightSize = t->right->size;

	t->size = leftSize + rightSize + 1;
    }
}