学步园

经典的二分，思路清晰，启发深刻

#include <iostream>
#include <fstream>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <vector>
#include <bitset>
#include <cmath>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <ctime>
#define LL long long
#define Vi vector<int>
#define Si set<int>
#define readf freopen("input.txt","r",stdin)
#define writef freopen("output.txt","w",stdout)
#define FU(i,a) for(int i(1); i <= (a); i++)
#define FD(i,a) for(int i(a); i >= (1); i--)
#define FOR(i,a,b) for(int i(a);i <= (b); i++)
#define FORD(i,a,b) for(int i(a);i >= (b); i--)
#define SET(a,b) memset(a,b,sizeof(a))
#define SD(a) scanf("%d",&(a))
#define LN printf("\n")
#define PS printf(" ")
#define pb push_back
#define lson l , m , rt << 1
#define rson m + 1 , r , rt << 1 | 1
using namespace std;
const double pi = acos(-1.0);
const int maxn = 10010;
const int INF=99999999;
int N,F;
double v[maxn];
bool judge(double k){
    int num=0;
    FOR(i,1,N){
        num+=int(v[i]/k);
        if(num>F)
            return true;
    }
    if(num>F)
        return true;
    return false;
}
int main() {
    int t;
    SD(t);
    while(t--){
        double sumV=0,left=0.0,right,mid;
        SD(N);SD(F);
        int r;
        FOR(i,1,N){
            SD(r);
            v[i]=pi*r*r;
            sumV+=v[i];
        }
        right=sumV/F;
        while((right-left)>1e-7){
            mid=(left+right)/2;
            if(judge(mid))
                left=mid;
            else
                right=mid;
        }
        printf("%.4lf\n",mid);
    }
    return 0;
}