bzoj 2784 [JLOI2012]时间流逝——树上高斯消元

#include<cstdio>
#include<cstring>
#include<algorithm>
#define mkp make_pair
#define fir first
#define sec second
#define db double
using namespace std;
const int N=35;
int n,m,w[N];db P;
pair<db,db> dfs(int lm,int s)
{
  db ta=0,tb=0;if(s>n)return mkp(0,0);
  for(int i=1;i<=lm;i++)
    {
      pair<db,db> v=dfs(i,s+w[i]);
      ta+=v.fir; tb+=v.sec;
    }
  if(!s)P=0;
  ta=1-(1-P)/lm*ta; ta=1/ta;
  tb=((1-P)/lm*tb+1)*ta; ta=P*ta;
  return mkp(ta,tb);
}
int main()
{
  while(scanf("%lf",&P)==1)
    {
      scanf("%d%d",&n,&m);
      for(int i=1;i<=m;i++)scanf("%d",&w[i]);
      sort(w+1,w+m+1);//
      printf("%.3f\n",dfs(m,0).sec);
    }
  return 0;
}