【指数型母函数】hdu1521 排列组合

#include<cstdio>
#include<cstring>
using namespace std;
int n,m,jiecheng[11];
double a[12],b[12];
int main()
{
	//freopen("hdu1521.in","r",stdin);
	int x;
	jiecheng[0]=1;
	for(int i=1;i<=10;++i)
	  jiecheng[i]=jiecheng[i-1]*i;
	while(scanf("%d%d",&n,&m)!=EOF)
	  {
	  	memset(a,0,sizeof(a));
	  	memset(b,0,sizeof(b));
	  	a[0]=1.0;
	  	for(int i=1;i<=n;++i)
	  	  {
	  	  	scanf("%d",&x);
	  	  	for(int j=0;j<=m;++j)
	  	  	  for(int k=0;k+j<=m && k<=x;++k)
	  	  	    b[k+j]+=a[j]/(double)jiecheng[k];
	  		memcpy(a,b,sizeof(a));
	  		memset(b,0,sizeof(b));
	  	  }
 
	  	printf("%.0lf\n",a[m]*(double)jiecheng[m]);
	  }
	return 0;
}

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排列组合

Time Limit: 2000/1000 MS (Java/Others)    Memory Limit: 65536/32768 K (Java/Others)
Total Submission(s): 3885    Accepted Submission(s): 1634

Problem Description

有n种物品，并且知道每种物品的数量。要求从中选出m件物品的排列数。例如有两种物品A,B，并且数量都是1，从中选2件物品，则排列有"AB","BA"两种。

 

Input

每组输入数据有两行，第一行是二个数n,m(1<=m,n<=10)，表示物品数，第二行有n个数，分别表示这n件物品的数量。

 

Output

对应每组数据输出排列数。(任何运算不会超出2^31的范围)

 

Sample Input

2 2
1 1

 

Sample Output

2

 

Author

xhd

 

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Administration

指数型母函数，题目很水，存个板子，日后也许可以用FFT再来一发。