求二叉树第K层的节点个数——递归

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