# hdu 1521 排列组合 （指数型母函数模板题）

Posted by 111qqz on Saturday, February 27, 2016

## TOC

http://acm.hdu.edu.cn/showproblem.php?pid=1521

``````/* ***********************************************
Author :111qqz
Created Time :2016年02月27日 星期六 19时48分31秒
File Name :code/hdu/1521.cpp
************************************************ */

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <string>
#include <cmath>
#include <cstdlib>
#include <ctime>
#define fst first
#define sec second
#define lson l,m,rt<<1
#define rson m+1,r,rt<<1|1
#define ms(a,x) memset(a,x,sizeof(a))
typedef long long LL;
#define pi pair < int ,int >
#define MP make_pair

using namespace std;
const double eps = 1E-8;
const int dx4[4]={1,0,0,-1};
const int dy4[4]={0,-1,1,0};
const int inf = 0x3f3f3f3f;
double a[15],tmp[15],f[15];
int n,m;
int num[15];
void pre()
{
f[0] = 1.0;
for ( int i = 1 ;i  <= 11 ; i++)
{
f[i] = f[i-1]*i*1.0;
}
}
int main()
{
#ifndef  ONLINE_JUDGE
freopen("code/in.txt","r",stdin);
#endif

pre();
while (scanf("%d %d",&n,&m)!=EOF)
{
for ( int i = 1 ; i <= n ; i++) scanf("%d",&num[i]);

ms(a,0);
ms(tmp,0);

for ( int i = 0 ; i <= num[1] ; i++)
{
a[i] = 1.0/f[i];
}

for ( int i = 2 ; i <= n ; i++)
{
for ( int j = 0 ; j <= m ; j++)
{
for ( int k = 0 ; k+j<=m&&k<=num[i] ; k++)
{
tmp[j+k] += a[j]/f[k];
}
}

for ( int j  = 0 ; j <= m ; j++)
{
a[j] = tmp[j];
tmp[j] = 0.0;
}
}

double ans = a[m]*f[m];
printf("%d\n",int(ans+0.5));
}

#ifndef ONLINE_JUDGE
fclose(stdin);
#endif
return 0;
}
``````

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