一道二分的題目，但要注意不能用double，
并且要注意一下二分的步驟
#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#define pi 3.1415926535898
#define eps 0.0001
using namespace std;
inline int read(){int x=0,f=1,ch=getchar();while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}return x*f;
}
inline long long max(long long x,long long y){return x<y?y:x;
}
int n,f;
long long a[10005];
inline bool ok(long long x){int res=0;for(int i=1;i<=n;i++) res+=a[i]/x;return res>f;
}
int main(){int T=read();while(T--){n=read(),f=read();long long maxn=0;for(int i=1;i<=n;i++) a[i]=read();for(int i=1;i<=n;i++) a[i]=a[i]*a[i]*pi*1000000;for(int i=1;i<=n;i++) maxn=max(maxn,a[i]);long long l=0,r=maxn;while(l<r){long long mid=(l+r)>>1;if(ok(mid+1)) l=mid+1;else r=mid;}printf("%.4lf\n",1.0*l/1000000);}
}
)
