求二叉树的第k层的节点个数

1.思路转换：左子树的k-1层和右子树的k-1层之和

2.返回条件：k==1且节点不为空或节点为空

int btreeksize(BTNode* point, int k)
{
    assert(k > 0);
    if (point == NULL)
    {
        return 0;
    }
    if (k == 1)
    {
        return 1;
    }
    return btreeksize(point->left, k - 1) + btreeksize(point->right, k - 1);
}