uva 11673 Garbage Remembering Exam （概率）

题目链接： http://vjudge.net/problem/viewProblem.action?id=42000

该过程为随即过程，因此总期望值等于个单词对应的期望值，即它们wasted的概率

 #include <stdio.h>
 #include <cstring>
 #include <cstdlib>
 #include <algorithm>
 #include <cmath>

 using namespace std;
 #define lson o<<1
 #define rson o<<1|1
 #define max(a,b) (a)>(b)?(a):(b)
 #define min(a,b) (a)<(b)?(a):(b)
 #define INF 200000000
 #define N 100010
 typedef long long ll;
 long double lg[N];

 void init (){
      lg[]=;
     for(int i=;i<N;i++){
        lg[i]=(long double)log(i)+lg[i-];
     }
 }
 int main(){
    int n,k,cs=;
    init();
    while(scanf("%d%d",&n,&k) && n){
       long double p,ans=;
       int up,down,put=*k;
       if(*k>n-)put=n-;
       long double l0=lg[n-]-lg[n--put];
       for(int i=;i<=n;i++){
           up=max(i--k,);
           down=max(n-i-k,);
           if(up+down>=put)p=exp(lg[up+down]-lg[up+down-put]-l0);
           else p=;
           ans+=-p;
       }
       printf("Case %d: %.4lf\n",cs++,(double)ans);
    }
    return ;
 }