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 =  + ;
 typedef long long ll;
 const double eps = 1e-;
 const double pi = acos(-1.0);
 double inf = ;
 
 struct circle
 {
     double x,y,r;
     void read()
     {
         scanf("%lf%lf%lf",&x,&y,&r);
     }
     double calS()
     {
         return pi*r*r;
     }
 }c[];
 
 double myabs(double x) {return x <  ? -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=;i<=n;i++)
     {
         if(get(now,c[i]) >= 0.5*c[i].calS()) ;
         else return ;
     }
     return ;
 }
 
 int main()
 {
     int T;
     scanf("%d",&T);
     while(T--)
     {
         scanf("%d",&n);
         for(int i=;i<=n;i++) c[i].read();
         double ans = inf;
         for(int i=;i<=n;i++)
         {
             circle now = c[i];
             double L = , R = inf;
             int CNT = ;
             while(CNT--)
             {
                 double mid = (L + R) / ;
                 now.r = mid;
                 if(solve(now)) R = mid;
                 else L = mid;
             }
             ans = min(ans, R);
         }
         printf("%.4f\n",ans);
     }
     return ;
 }