1 #include<stdio.h>
2 #include<string.h>
3 #include<algorithm>
4 using namespace std;
5 #define eps 1e-9
6 long long sum1[50005],sum2[50005];
7 int a[50005];
8 int main()
9 {
10 int T,i,n,k;
11 double pre,now;
12 scanf("%d",&T);
13 while (T--)
14 {
15 scanf("%d%d",&n,&k);
16 for (i=1;i<=n;i++) scanf("%d",&a[i]);
17 sort(a+1,a+n+1);
18 sum1[0]=sum2[0]=0;
19 for (i=1;i<=n;i++)
20 {
21 sum1[i]=sum1[i-1]+a[i];
22 sum2[i]=sum2[i-1]+(long long )a[i]*(long long)a[i];
23 }
24 if (k==n||k==n-1) {printf("0\n"); continue; }
25 k=n-k;
26 pre=(1.0*sum2[k]-1.0*sum1[k]*sum1[k]/(k))/k;
27 for (i=k+1;i<=n;i++)
28 {
29 now=(1.0*(sum2[i]-sum2[i-k])-1.0*(sum1[i]-sum1[i-k])*(sum1[i]-sum1[i-k])/(k))/k;
30 if (pre-now>eps) pre=now;
31 }
32 printf("%lf\n",pre*k);
33 }
34 }
http://acm.hdu.edu.cn/showproblem.php?pid=5073
