Pie

Time Limit: 5000/1000 MS (Java/Others)    Memory Limit: 65536/32768 K (Java/Others)
Total Submission(s): 6564    Accepted Submission(s): 2480


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題目大意：主人家裡來了F個他的朋友，他家裡有好幾個Pie，主人希望把Pie分出F+1份，體積一樣的（主人自己也要一個），所有的Pie不需要都分完，問你每個人最大能分到多大體積的Pie【程式碼】

#include<stdio.h>
#include<algorithm>
#include<math.h>
#define PI acos(-1.0)       //PI儘量用反餘弦函式來算，否則可能精度不夠
using namespace std;
int n,f;
int a[10010];
bool s(double x)
{
	int sum=0;
	for(int i=0;i<n;i++)
	  sum+=int(a[i]*a[i]/(x*x));
	return sum>=f+1;
}
int main()
{
	int i,t;
	double v;
	scanf("%d",&t);
	while(t--)
	{
		scanf("%d%d",&n,&f);
		for(i=0;i<n;i++)
		  scanf("%d",&a[i]);
		double l=0,r=10000,mid;
		while(r-l>1e-10)
		{
			mid=(l+r)/2;
			if(s(mid)) l=mid;
			else r=mid;
		}
		v=PI*r*r;
		printf("%.4lf\n",v);
	}
	return 0;
	
}