HDU 1521（排列组合）

母函数题，求排列使用指数型母函数，套用母函数模板即可。

#include 
#include 
using namespace std;
const int MAXN = 15;

int fac[MAXN]; //阶乘
int num[MAXN];
double c1[MAXN], c2[MAXN];

//计算10以内的阶乘
void getFac()
{
	fac[0] = fac[1] = 1;
	for (int i = 2; i <= 10; i++)
	{
		fac[i] = fac[i - 1] * i;
	}
}

int main()
{
	getFac();
	int n, m;
	while (cin >> n >> m)
	{
		for (int i = 1; i <= n; i++)
		{
			cin >> num[i];
		}
		for (int i = 0; i <= 10; i++)
		{
			c1[i] = c2[i] = 0.0;
		}
		for (int i = 0; i <= num[1]; i++) //第一个括号中的系数
		{
			c1[i] = 1.0 / fac[i];
		}
		for (int i = 2; i <= n; i++) //母函数模板
		{
			for (int j = 0; j <= m; j++)
			{
				for (int k = 0; k <= num[i] && k + j <= m; k++)
					c2[k + j] += c1[j] / fac[k];
			}
			for (int j = 0; j <= m; j++)
			{
				c1[j] = c2[j];
				c2[j] = 0;
			}
		}
		cout << fixed << setprecision(0) << c1[m] * fac[m] << endl;
	}
	return 0;
}

继续加油。