cf 261B.Maxim and Restaurant

什么什么期望的，不会！

（题解http://blog.sina.com.cn/s/blog_140e100580102wj4e.html（看不懂））

#include<bits/stdc++.h>
#define LL long long
#define LD long double
#define  N 100005
using namespace std;
inline int ra()
{
    int x=,f=; char ch=getchar();
    while (ch<'' || ch>'') {if (ch=='-') f=-; ch=getchar();}
    while (ch>='' && ch<='') {x=x*+ch-''; ch=getchar();}
    return x*f;
}
LD C[];
LD dp[][][];
int a[];
int main()
{
    int n=ra();
    C[]=;
    for (int i=; i<=n; i++)
    {
        C[i]=C[i-]*(LD)i;
        a[i]=ra();
    }
    sort(a+,a+n+);
    int m=ra();
    dp[][][]=; dp[][][a[]]=;
    for (int i=; i<=n; i++)
        for (int j=; j<=i; j++)
            for (int k=; k<=m; k++)
            {
                dp[i+][j+][k+a[i+]]+=dp[i][j][k];
                dp[i+][j][k]+=dp[i][j][k];
            }
    LD res=;
    for (int j=; j<=n; j++)
        for (int k=; k<=m; k++)
        {
            res+=dp[n][j][k]*(C[j]*C[n-j]);
            cout<<j<<"  "<<k<<"   "<<dp[n][j][k]<<endl;
        }
    res=res/C[n];
    cout<<res;
    return ;
}