POJ 2443 Set Operation -- 位运算 + bitset

/* http://poj.org/problem?id=2443 POJ2443 Set Operation -- 位运算 + bitset
询问两个元素在不在同一个集合内
(在 ? yes : no)
*/

#include <iostream>
#include <cstring>
#include <cstdio>
#include <algorithm>
#include <cmath>
using namespace std;
#define CLR(c,v) memset(c,v,sizeof(c));
int const N = 1e5 +10;

double loge
;
double nCr(int n,int r){
if(n==0)return 0.;
return loge
- loge[r] - loge[n-r];
}
int a
,sum
;

int main(){
//freopen("in.txt","r",stdin);
loge[0] = 0;
for(int i = 1 ; i < N ; i++)
loge[i] += loge[i-1] + log(i*1.0);

int n,k;
while(cin >> n >> k,n||k){
CLR(sum,0);
for(int i = 0 ; i < n ; i++){
scanf("%d",&a[i]);
sum[i+1] += sum[i] + a[i];
}
double ans = 0;
for(int i = k ; i <= n ; i++){ // seq length
double p = exp( nCr(i-2,k-2) - nCr(n,k) );
ans += p * ( (sum
-sum[i-1])-(sum[n-i+1]-sum[0]) );
}
printf("%.3f\n" , ans);
}
return 0;
}