fzu1715BallandBoxn个不同的求放到m个不同的盒子中方法的个数

1715 Ball and Box

Accept: 120    Submit: 288
Time Limit: 1000 mSec    Memory Limit : 32768 KB

 Problem Description

CDGG has r different boxes and n different balls.He want to put the n balls into the r boxes,and it is not allow to left empty box.How many ways can CDGG put the n balls into the r boxes?

 Input

The input consists of several test cases. For each case,the only line contains two integer n and r,indicates the number of balls and the number of boxes.(0<=n,r<=10)

 Output

For each case,output a line contain a integer,indicates the answer.

 Sample Input

3 2

 Sample Output

6 #include#include#includeusing namespace std;//第二类斯特林数void Stirling_2(__int64 a[][25],int n){    //注意:不能在这里写memset(a..);这样不能对main函数中的a数组初始化    //memset(a,0,sizeof(a));    a[1][1]=1;    for(int i=2;i<=n;i++)    {        for(int j=1;j<=i;j++)        {            a[i][j]=j*a[i-1][j]+a[i-1][j-1];        }    }}int main(){        __int64 a[25][25],fac[25]={0,1};    memset(a,0,sizeof(a));    Stirling_2(a,20);    for(int i=2;i<=20;i++) fac[i]=fac[i-1]*i;    int n,m;    while(scanf("%d%d",&n,&m)==2)    {        __int64 cnt=a[n][m]*fac[m];//important        printf("%I64d/n",cnt);    }    return 0;}