【概率DP】51Nod 1398 等公交

題面在這裏

考慮fi 表示用了i時間，用了任意輛車概率之和

然後就好了

示例程序：

#include<cstdio>
#include<cstring>
#define cl(x,y) memset(x,y,sizeof(x))
inline char nc(){
    static char buf[100000],*p1=buf,*p2=buf;
    return p1==p2&&(p2=(p1=buf)+fread(buf,1,100000,stdin),p1==p2)?EOF:*p1++;
}
inline int red(){
    int res=0,f=1;char ch=nc();
    while (ch<'0'||'9'<ch) {if (ch=='-') f=-f;ch=nc();}
    while ('0'<=ch&&ch<='9') res=res*10+ch-48,ch=nc();
    return res*f;
}

const int maxs=200005,maxn=105;
int tst,n,s,t[maxn];
double p[maxn],f[maxs];
int main(){
    tst=red();
    while (tst--){
        n=red(),s=red();
        for (int i=1;i<=n;i++)
         t[i]=red(),p[i]=red()/100.0;
        cl(f,0);
        f[0]=1;double ans=0;
        for (int i=0;i<s;i++)
         for (int j=1;j<=n;j++)
          f[i+t[j]]+=f[i]*p[j];
        for (int i=s+1;i<=200000;i++)
         ans+=(i-s)*f[i];
        printf("%.4lf\n",ans);
    }
    return 0;
}