hdu4336
容斥原理模板:dfs部分
//hdu4336
#include<bits/stdc++.h>
using namespace std;
#define out(x) cout<<#x<<": "<<x<<endl
const double eps(1e-8);
const int maxn=10100;
const long long maxint=-1u>>1;
const long long maxlong=maxint*maxint;
typedef long long lint;
int n;
double p[maxn],ans;
void init()
{
for (int i=1; i<=n; i++)
cin>>p[i];
}
void dfs(int x, int tot, double sum)
{
if (x==n+1)
{
if (sum==0.0) return;//divide by zero is illegal
if (tot&1)
ans+=1/sum;
else
ans-=1/sum;
return;
}
dfs(x+1,tot,sum);
dfs(x+1,tot+1,sum+p[x]);
}
void work()
{
ans=0;
dfs(1,0,0.0);
printf("%.4f\n",ans);
}
int main()
{
while(cin>>n)
{
init();
work();
}
return 0;
}