寒假训练HDU-2049题解（错排）

问题链接：HDU-2049

思路

组合数加错排问题，答案为答对新娘的组合数和打错新娘的错排数的乘积。注意要用long long int.

AC的代码

#include
using namespace std;
long long int f(long long int );
long long int g(long long int );
int main()
{
	int n;
	cin >> n;
	for (int i = 0; i < n; i++)
	{
		long long int a, b;
		cin >> a >> b;
		if (a > b) cout << (g(a) / (g(b)*g(a - b)))*f(b) << endl;
		else cout << f(b) << endl;
	}
	return 0;
}
long long int f(long long int x)
{
	if (x == 1) return 0;
	if (x == 2) return 1;
	return (x - 1)*(f(x - 1) + f(x - 2));
}
long long int g(long long int x)
{
	if (x == 1)return 1;
	else return x * g(x - 1);
}