BZOJ1426: 收集邮票

#include<iostream>
#include<cstring>
#include<cstdio>
#include<cmath>
#include<algorithm>
#define N 10005
using namespace std;
int n;
double f[N],g[N];
int main()
{
    scanf("%d",&n);
    f[n]=g[n]=0;
    for (int i=n-1; i>=0; i--)
    {
        f[i]=f[i+1]+1.0*n/(n-i);
        g[i]=g[i+1]+2.0*f[i+1]+2.0*i/(n-i)*f[i]+1.0*n/(n-i); 
     } 
    printf("%0.2lf",(f[0]+g[0])/2);
    return 0;
}