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

思路：指數型母函式模板題。

#include<iostream>
#include<string.h>
#include<stdio.h>
using namespace std;
int jc(int s)
{
    int ans=1;
    for (int i = 1; i <= s; i++)
        ans *= i;
    return ans;
}
int main()
{
    int a[15];double c1[15], c2[15];
    int m, n,i,j;
    while (cin >> n >> m)
    {
        memset(c1, 0, sizeof(c1));
        memset(c2, 0, sizeof(c2));
        for (i = 1; i <= n; i++)
            cin >> a[i];
        for (i = 0; i <= a[1]; i++)
            c1[i] = 1.0 / jc(i);//先把分母也當做係數包含進去
        for (i = 2; i <= n; i++)
        {
            for(j=0;j<=m;j++)
                for (int k = 0; k + j <= m&&k <= a[i]; k++)
                    c2[j + k] +=1* c1[j]/jc(k);//把第k項係數包含進去，如x^3/3!*x^2/2!
            for (j = 0; j <= m; j++)
            {
                c1[j] = c2[j];
                c2[j] = 0;
            }

        }
        printf("%.0f\n", c1[m]*jc(m));
    }
    return 0;
}