判断是否平衡二叉树

bool BinTree::is_avl(BintreeNode *r, int& height)
{
	if (r == NULL)
	{
		height = 0;
		return true;
	}

	int hl,hr;
	bool is_avl_left = is_avl(r->get_left(), hl);
	if ( !is_avl_left )
		return false;

	bool is_avl_right = is_avl(r->get_right(), hr);
	if ( !is_avl_right )
		return false;

	int max = hl>hr ? hl:hr;
	if (2*max-hr-hl > 2)
		return false;

	height = max + 1;
	return true;
}