套模板
不知道有没有x,y=0情况，不过这种情况都按+号输出的。
1 #include <iostream> 2 #include<cstdio> 3 #include<cstring> 4 #include<algorithm> 5 #include<stdlib.h> 6 #include<vector> 7 #include<cmath> 8 #include<queue> 9 #include<set> 10 using namespace std; 11 #define N 100000 12 #define LL long long 13 #define INF 0xfffffff 14 const double eps = 1e-8; 15 const double pi = acos(-1.0); 16 const double inf = ~0u>>2; 17 struct Point 18 { 19 double x,y; 20 Point (double x=0,double y =0):x(x),y(y){} 21 }p[4]; 22 struct Circle 23 { 24 Point center; 25 double r; 26 }; 27 typedef Point pointt; 28 pointt operator - (Point a,Point b) 29 { 30 return Point(a.x-b.x,a.y-b.y); 31 } 32 int dcmp(double x) 33 { 34 if(fabs(x)<eps) return 0; 35 return x<0?-1:1; 36 } 37 double dis(Point a) 38 { 39 return a.x*a.x+a.y*a.y; 40 } 41 double cross(Point a,Point b) 42 { 43 return a.x*b.y-a.y*b.x; 44 } 45 double area() 46 { 47 return fabs(cross(p[1]-p[2],p[2]-p[3]))/2; 48 } 49 struct Circle Circumcircle() 50 { 51 Circle tmp; 52 double a,b,c,c1,c2; 53 double xa,ya,xb,yb,xc,yc; 54 a = sqrt(dis(p[3]-p[1])); 55 b = sqrt(dis(p[1]-p[2])); 56 c = sqrt(dis(p[2]-p[3])); 57 //根据s = a*b*c/R/4，求半径 58 tmp.r = (a*b*c)/(area()*4.0); 59 xa = p[3].x; 60 ya = p[3].y; 61 xb = p[1].x; 62 yb = p[1].y; 63 xc = p[2].x; 64 yc = p[2].y; 65 c1 = (dis(p[3])-dis(p[1]))/2; 66 c2 = (dis(p[3])-dis(p[2]))/2; 67 tmp.center.x = (c1*(ya-yc)-c2*(ya-yb))/((xa-xb)*(ya-yc)-(xa-xc)*(ya-yb)); 68 tmp.center.y = (c1*(xa-xc)-c2*(xa-xb))/((ya-yb)*(xa-xc)-(ya-yc)*(xa-xb)); 69 return tmp; 70 } 71 int main() 72 { 73 while(scanf("%lf%lf%lf%lf%lf%lf",&p[1].x,&p[1].y,&p[2].x,&p[2].y,&p[3].x,&p[3].y)!=EOF) 74 { 75 Circle cc = Circumcircle(); 76 double r = cc.r; 77 char c1,c2; 78 c1 = dcmp(cc.center.x)>0?'-':'+'; 79 c2 = dcmp(cc.center.y)>0?'-':'+'; 80 printf("(x %c %.3f)^2 + (y %c %.3f)^2 = %.3f^2 ",c1,fabs(cc.center.x),c2,fabs(cc.center.y),r); 81 double s = r*r-cc.center.x*cc.center.x-cc.center.y*cc.center.y; 82 char c3 = dcmp(s)>0?'-':'+'; 83 printf("x^2 + y^2 %c %.3fx %c %.3fy %c %.3f = 0 ",c1,fabs(cc.center.x*2),c2,fabs(cc.center.y*2),c3,fabs(s)); 84 puts(""); 85 } 86 return 0; 87 }