转态转移非常好想
状态压缩一下。然后倒着转移
#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
using namespace std;
double dp[1<<21];
int n;
double p[21];
double fai=1;
int main(){
while(~scanf("%d",&n)){
fai=1;
memset(dp,0,sizeof(dp));
for(int i=1;i<=n;++i){
scanf("%lf",&p[i]);
fai-=p[i];
}
for(int i=(1<<n)-2;i>=0;--i){
double x=0;
double y=fai;
for(int j=1;j<=n;++j){
if((i&(1<<(j-1)))){
y+=p[j];
}else{
x+=p[j]*(1+dp[i|(1<<(j-1))]);
}
}
dp[i]=(x+y)/(1-y);
}
printf("%.4lf
",dp[0]);
}
return 0;
}