hdu 4720

最小覆盖圆的模板；

 #include<stdio.h>
 #include<string.h>
 #include<math.h>
 struct Point
 {
     double x;
     double y;
 } pt[];
 struct Traingle
 {
     struct Point p[];
 };
 struct Circle
 {
     struct Point center;
     double r;
 } ans;
 //计算两点距离
 double Dis(struct Point p, struct Point q)
 {
     double dx=p.x-q.x;
     double dy=p.y-q.y;
     return sqrt(dx*dx+dy*dy);
 }
 //计算三角形面积
 double Area(struct Traingle ct)
 {
     return
         fabs((ct.p[].x-ct.p[].x)*(ct.p[].y-ct.p[].y)-(ct.p[].x-ct.p[].x)*(ct.p[].y-ct.p[].y))/2.0;
 }
 //求三角形的外接圆，返回圆心和半径(存在结构体"圆"中)
 struct Circle CircumCircle(struct Traingle t)
 {
     struct Circle tmp;
     double a, b, c, c1, c2;
     double xA, yA, xB, yB, xC, yC;
     a = Dis(t.p[], t.p[]);
     b = Dis(t.p[], t.p[]);
     c = Dis(t.p[], t.p[]);
 //根据S = a * b * c / R / 4;求半径R
     tmp.r = (a*b*c)/(Area(t)*4.0);
     xA = t.p[].x;
     yA = t.p[].y;
     xB = t.p[].x;
     yB = t.p[].y;
     xC = t.p[].x;
     yC = t.p[].y;
     c1 = (xA*xA+yA*yA - xB*xB-yB*yB) / ;
     c2 = (xA*xA+yA*yA - xC*xC-yC*yC) / ;
     tmp.center.x = (c1*(yA - yC)-c2*(yA - yB)) / ((xA - xB)*(yA - yC)-(xA - xC)*(yA - yB));
     tmp.center.y = (c1*(xA - xC)-c2*(xA - xB)) / ((yA - yB)*(xA - xC)-(yA - yC)*(xA - xB));
     return tmp;
 }
 //确定最小包围圆
 struct Circle MinCircle(int num, struct Traingle ct)
 {
     struct Circle ret;
     if (num==) ret.r = 0.0;
     else if (num==)
     {
         ret.center = ct.p[];
         ret.r = 0.0;
     }
     else if (num==)
     {
         ret.center.x = (ct.p[].x+ct.p[].x)/2.0;
         ret.center.y = (ct.p[].y+ct.p[].y)/2.0;
         ret.r = Dis(ct.p[], ct.p[])/2.0;
     }
     else if(num==) ret = CircumCircle(ct);
     return ret;
 }
 //递归实现增量算法
 void Dfs(int x, int num, struct Traingle ct)
 {
     int i, j;
     struct Point tmp;
     ans = MinCircle(num, ct);
     if (num==) return;
     for (i=; i<=x; i++)
         if (Dis(pt[i], ans.center)>ans.r)
         {
             ct.p[num]=pt[i];
             Dfs(i-, num+, ct);
             tmp=pt[i];
             for (j=i; j>=; j--)
                 pt[j]=pt[j-];
             pt[]=tmp;
         }
 }
 void Solve(int n)
 {
     struct Traingle ct;
     Dfs(n, , ct);
 }
 int main (void)
 {
     int n, i,t;
     int ca=;
     scanf("%d",&t);
     while (t--)
     {
         for (i=; i<=; i++)
             scanf("%lf %lf", &pt[i].x, &pt[i].y);
         Solve();
         double xx,yy;
         scanf("%lf%lf",&xx,&yy);
         printf("Case #%d: ",ca++);
         if((xx-ans.center.x)*(xx-ans.center.x)+(yy-ans.center.y)*(yy-ans.center.y)<=(ans.r)*(ans.r))
             puts("Danger");
         else puts("Safe");
     }
     return ;
 }