POJ 1329 Circle Through Three Points(三角形外接圆)

题目链接：http://poj.org/problem?id=1329

 #include<cstdio>
 #include<cmath>
 #include<algorithm>
 #include<iostream>
 #include<cstring>
 using namespace std;
 typedef long long ll;
 const double eps = 1e-;
 int sgn(double x)
 {
     if(fabs(x) < eps) return ;
     else return x <  ? - : ;
 }
 struct Point{
     double x, y;
     Point(){}
     Point(double _x, double _y){
         x = _x, y = _y;
     }
     bool operator == (Point b) const{
         return sgn(x - b.x) ==  && sgn(y - b.y) == ;
     }
     bool operator < (Point b)const{
         return sgn(x - b.x) ==  ? sgn(y - b.y < ) : x < b.x;
     }
     Point operator - (const Point &b)const{
         return Point(x - b.x, y - b.y);
     }
     //叉积
     double operator ^(const Point &b){
         return x * b.y - y * b.x;
     }
     //点积
     double operator *(const Point &b){
         return x * b.x + y * b.y;
     }
     double len(){
         return hypot(x, y);
     }
     double len2(){
     return x * x + y * y;
     }
     double distant(Point p){
         return hypot(x - p.x, y - p.y);
     }
     Point operator + (const Point &b)const{
         return Point (x + b.x, y + b.y);
     }
     Point operator * (const double &k)const{
         return Point(x * k, y * k);
     }
     Point operator / (const double &k)const{
         return Point(x / k, y / k);
     }
     //逆时针旋转90度
     Point rotleft(){
         return Point(- y, x);
     }
     //顺时针旋转90度
     Point rotright(){
         return Point(y, -x);
     }
 };
 struct Line{
     Point s, e;
     Line(){}
     Line(Point _s, Point _e){s = _s, e = _e;}
     Point crosspoint(Line v){
         double a1 = (v.e - v.s)^(s - v.s);
         double a2 = (v.e - v.s)^(e - v.s);
         return Point((s.x*a2 - e.x*a1)/(a2 - a1),(s.y*a2 - e.y*a1)/(a2 - a1));
     }
 };
 struct circle{
     Point p;
     double r;
     circle(){}
     circle(Point _p, double _r){
         p = _p, r = _r;
     }
     circle(double x, double y, double _r){
         p = Point(x, y);
         r = _r;
     }
     //三角形外接圆
     circle(Point a, Point b, Point c){
         Line u = Line( (a + b) / ,((a + b) / ) + ((b - a).rotleft()) );
         Line v = Line( (b + c) / ,((b + c) / ) + ((c - b).rotleft()) );
         p = u.crosspoint(v);
         r = p.distant(a);
     }
 };
 void print(double num) {
     if (sgn(num) < ) {
             printf("- "); num = -num;
     }
     else printf("+ ");
     printf("%.3f", num);
 }
 int main()
 {
     double x1, x2, x3, y1, y2, y3;
     while(~scanf("%lf%lf%lf%lf%lf%lf",&x1, &y1, &x2, &y2, &x3, &y3))
     {
         circle a = circle(Point(x1, y1), Point(x2, y2), Point(x3, y3));
         double c = a.r * a.r - (a.p.x * a.p.x + a.p.y * a.p.y);
         if (sgn(a.p.x) == )printf("x^2");
         else { printf("(x "); print(-a.p.x); printf(")^2"); }
         printf(" + ");
         if (sgn(a.p.y) == )printf("y^2");
         else { printf("(y "); print(-a.p.y); printf(")^2"); }
         printf(" = %.3f^2\n", a.r);
         printf("x^2 + y^2 ");
         print(-a.p.x * 2.0);
         printf("x ");
         print(-a.p.y * 2.0);
         printf("y ");
         print(-c);
         printf(" = 0\n");
         printf("\n");
     }
     return ;
 }