[概率dp] hdu 4336 Card Collector

代码

#include<iostream>
#include<cstdio>
#include<cstring>
#include<string>
#include<cmath>
#include<map>
#include<algorithm>
using namespace std;
const double eps=1e-9;
const int maxn=20;
const int maxs=(1<<maxn)+5;
double p[maxn],dp[maxs];
int num[maxs]={0};
int n;
int main()
{
    for(int i=0;i<(1<<20);i++)
        {
            int x=i;
            while(x>0)
            {
                num[i]+=(x&1);
                x>>=1;
            }
        }
    while(cin>>n)
    {
        p[0]=1;
        for(int i=1;i<=n;i++) scanf("%lf",&p[i]),p[0]-=p[i];
        dp[(1<<n)-1]=0;

        for(int len=n-1;len>=0;len--)
        {
            for(int s=0;s<(1<<n);s++)
            {
                if(num[s]!=len) continue;
                double  x=p[0],y=1.0;
                for(int dig=0;dig<n;dig++)
                    if((s>>dig)&1)
                        x+=p[dig+1];
                    else
                        y+=p[dig+1]*dp[(s^(1<<dig))];
                x=1-x;
                if(fabs(x)<eps) {dp[s]=0;continue;}
                dp[s]=y/x;
            }
        }
        printf("%.9f\n",dp[0]);
    }
    return 0;
}