bzoj 2784: [JLOI2012]时间流逝树形期望dp

Posted 2021-01-06 lokiii

来自lyd课件
发现s和last(s),next(s)成树结构，然后把式子化简成kx+b的形式，做树形dp即可

#include<iostream>
#include<cstdio>
#include<algorithm>
using namespace std;
int n,t,a[105];
double p,q;
struct qwe
{
    double k,b;
    qwe(double K=0,double B=0)
    {
        k=K,b=B;
    }
};
qwe dfs(int s,int mx)
{
    if(s>t) 
        return qwe(0,0);    
    double ne,b=0,k=0,r=1;
    ne=1.0/(double)mx;
    for(int i=1;i<=mx;i++)
    {
        qwe nw=dfs(s+a[i],i);
        b=b+nw.b,k=k+nw.k;
    }
    q=s?p:0.0;
    r=1.0-(1.0-q)*ne*k;
    k=q/r;
    b=((1.0-q)*ne*b+1)/r;
    return qwe(k,b);
}
int main()
{
    while(~scanf("%lf%d%d",&p,&t,&n))
    {
        for(int i=1;i<=n;i++) 
            scanf("%d",&a[i]);
        sort(a+1,a+n+1);
        printf("%.3lf
",dfs(0,n).b);
    }
    return 0;
}