bzoj4008

题解：

递推

然后再加上概率的计算

代码：

#include<bits/stdc++.h>
using namespace std;
const int N=255;
int T,n,r,d[N];
double p[N],f[N][N],pw[N][N];
int main()
{
    scanf("%d",&T);
    while (T--)
     {
        scanf("%d%d",&n,&r);
        memset(f,0,sizeof(f));
        for (int i=1;i<=n;i++)scanf("%lf%d",&p[i],&d[i]);
        double ans=0;
        for (int i=1;i<=n;i++)
         {
            pw[i][0]=1;
            for (int j=1;j<=r;j++)pw[i][j]=pw[i][j-1]*(1-p[i]);
         }
        f[0][r]=1;
        for (int i=0;i<n;i++)
         for (int j=0;j<=r;j++)
          {
            f[i+1][j]+=f[i][j]*pw[i+1][j];
            if (j-1>=0)
             {
                f[i+1][j-1]+=f[i][j]*(1-pw[i+1][j]);
                ans+=f[i][j]*(1-pw[i+1][j])*d[i+1];
             }
          }
        printf("%.10lf\n",ans);
     }
    return 0;
}