HDU 3264 Open-air shopping malls ——（二分+圆交）

　　纯粹是为了改进牛吃草里的两圆交模板= =。

　　代码如下：

#include <stdio.h>
#include <algorithm>
#include <string.h>
#include <vector>
#include <math.h>
using namespace std;
const int N = 100000 + 100;
typedef long long ll;
const double eps = 1e-8;
const double pi = acos(-1.0);
double inf = 50000;

struct circle
{
    double x,y,r;
    void read()
    {
        scanf("%lf%lf%lf",&x,&y,&r);
    }
    double calS()
    {
        return pi*r*r;
    }
}c[30];

double myabs(double x) {return x < 0 ? -x : x;}

double get(circle c1,circle c2)
{
    double a = c1.x, b = c1.y, R = c1.r;
    double x = c2.x, y = c2.y, r = c2.r;
    double dx = myabs(a-x), dy = myabs(b-y);
    double d = sqrt(dx*dx+dy*dy);
    if(d > R + r) return 0.0;
    if(R < r) swap(R,r);
    if(d < R-r) return pi*r*r;
    double A = 2.0*acos((R*R+d*d-r*r)/(2.0*R*d));
    double B = 2.0*acos((r*r+d*d-R*R)/(2.0*r*d));
    double s1 = 0.5*A*R*R + 0.5*B*r*r;
    double s2 = 0.5*R*R*sin(A) + 0.5*r*r*sin(B);
    return s1 - s2;
}

int n;
bool solve(circle now)
{
    for(int i=1;i<=n;i++)
    {
        if(get(now,c[i]) >= 0.5*c[i].calS()) ;
        else return 0;
    }
    return 1;
}

int main()
{
    int T;
    scanf("%d",&T);
    while(T--)
    {
        scanf("%d",&n);
        for(int i=1;i<=n;i++) c[i].read();
        double ans = inf;
        for(int i=1;i<=n;i++)
        {
            circle now = c[i];
            double L = 0, R = inf;
            int CNT = 150;
            while(CNT--)
            {
                double mid = (L + R) / 2;
                now.r = mid;
                if(solve(now)) R = mid;
                else L = mid;
            }
            ans = min(ans, R);
        }
        printf("%.4f
",ans);
    }
    return 0;
}