hdu4336 Card Collector

 1 #include<cstdio>
 2 
 3 int main()
 4 {
 5     int n;
 6     double a[21];
 7     while (~scanf("%d",&n))
 8     {
 9         for (int i=0;i<n;++i)
10             scanf("%lf",&a[i]);
11         double temp,ans=0;
12         for (int i=1;i< (1<<n);++i)
13         {
14             temp=0;
15             int ct=0;
16             for (int j=0;j<n;++j)
17               if (i & (1<<(j) ) )
18               {
19                   ct++;
20                   temp+=a[j];
21               }
22             if (ct & 1) ans+=1/temp;
23             else ans-=1/temp;
24         }
25         printf("%.4f\n",ans);
26     }    
27     return 0;
28 }