HDU 4336 Card Collector

#pragma comment(linker, "/STACK:1024000000,1024000000")
#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
#include<vector>
#include<map>
#include<set>
#include<queue>
#include<stack>
#include<iostream>
using namespace std;
typedef long long LL;
const double pi=acos(-1.0),eps=1e-6;
void File()
{
    freopen("D:\\in.txt","r",stdin);
    freopen("D:\\out.txt","w",stdout);
}
template <class T>
inline void read(T &x)
{
    char c = getchar();
    x = 0;
    while(!isdigit(c)) c = getchar();
    while(isdigit(c))
    {
        x = x * 10 + c - ‘0‘;
        c = getchar();
    }
}

int n;
double p[30];
double dp[(1<<20)+10],P;

int main()
{
    while(~scanf("%d",&n))
    {
        P=0;
        for(int i=0;i<n;i++)
        {
            scanf("%lf",&p[i]);
            P=P+p[i];
        }
        for(int i=0;i<(1<<n);i++) dp[i]=0;

        for(int i=(1<<n)-2;i>=0;i--)
        {
            double sum1=0,sum2=0;
            for(int j=0;j<n;j++)
            {
                if((1<<j)&i) sum1=sum1+p[j];
                else sum2=sum2+dp[(1<<j)+i]*p[j];
            }
            sum2=sum2+1.0;
            sum1=sum1+1.0-P;
            dp[i]=sum2/(1.0-sum1);
        }

        printf("%.4f\n",dp[0]);
    }
    return 0;
}