1003

原题
Problem Description
My birthday is coming up and traditionally I’m serving pie. Not just one pie, no, I have a number N of them, of various tastes and of various sizes. F of my friends are coming to my party and each of them gets a piece of pie. This should be one piece of one pie, not several small pieces since that looks messy. This piece can be one whole pie though.

My friends are very annoying and if one of them gets a bigger piece than the others, they start complaining. Therefore all of them should get equally sized (but not necessarily equally shaped) pieces, even if this leads to some pie getting spoiled (which is better than spoiling the party). Of course, I want a piece of pie for myself too, and that piece should also be of the same size.

What is the largest possible piece size all of us can get? All the pies are cylindrical in shape and they all have the same height 1, but the radii of the pies can be different.

Input
One line with a positive integer: the number of test cases. Then for each test case:
—One line with two integers N and F with 1 <= N, F <= 10 000: the number of pies and the number of friends.
—One line with N integers ri with 1 <= ri <= 10 000: the radii of the pies.

Output
For each test case, output one line with the largest possible volume V such that me and my friends can all get a pie piece of size V. The answer should be given as a floating point number with an absolute error of at most 10^(-3).

Sample Input
3
3 3
4 3 3
1 24
5
10 5
1 4 2 3 4 5 6 5 4 2

Sample Output
25.1327
3.1416
50.2655

大意：
若干个朋友分若干个大小不一样的pie。每个朋友只能分到一块，求能每个人能分到的最大面积。

思路形成：
pie的大小有几个范围是0到最大的面积。适用二分。
当面积小的时候能分的朋友多，面积大的时候能分的朋友少，目标是能分的人数与实际朋友数一样

#include<stdio.h>
#include<math.h>
#define eps 1e-7
#define pi 3.14159265358979

int main(void)
{
// freopen("nrj.txt","r",stdin);
    int t,n,f;
    double s[10005],r;
    scanf("%d",&t);
    while(t--)
    {
        double up=0,low=0;
        scanf("%d%d",&n,&f);
        f+=1;
        for(int i=0;i<n;i++)
        {
            scanf("%lf",&r);
            s[i]=pi*r*r;
            if(s[i]>up) up=s[i];
          }
        double mid=(up+low)/2;
        while(up-low>eps)
        {
            int cnt=0;
            for(int i=0;i<n;i++)
                cnt+=(int)(s[i]/mid);
            if(cnt>=f) low=mid;
            else up=mid; 
            mid=(up+low)/2;
        }
        printf("%.4lf\n",mid);

    }
     return 0;
}