【HDU 5733】tetrahedron

#include <cstdio>
#include <cmath>
#define dd double
struct point{
    dd x,y,z;
    int input(){
        return scanf("%lf%lf%lf",&x,&y,&z);
    }
    point operator -(const point &b) const
    {
        return (point){x-b.x,y-b.y,z-b.z};
    }
    dd operator *(const point &b)const
    {
        return x*b.x+y*b.y+z*b.z;
    }
    point operator ^(const point &b)const
    {
        return (point){y*b.z-b.y*z,b.x*z-x*b.z,x*b.y-b.x*y};
    }
}p[5];
dd sqr(dd x){
    return x*x;
}
dd area(const point &o,const point &s,const point &e,point &n){
    point a=s-o,b=e-o;
    n=a^b;
    return sqrt(sqr(n.x)+sqr(n.y)+sqr(n.z))/2;
}
int main() {
    while(~p[1].input()){
        for(int i=2;i<=4;i++)
            p[i].input();
        dd s[5];
        point n,l=p[1]-p[2];;
        s[1]=area(p[2],p[3],p[4],n);
        if(l*n==0){
            puts("O O O O");
            continue;
        }
        s[2]=area(p[1],p[3],p[4],n);
        s[3]=area(p[1],p[2],p[4],n);
        s[4]=area(p[1],p[2],p[3],n);
        dd x=0,y=0,z=0,down=0;
        for(int i=1;i<=4;i++){
            down+=s[i];
            x+=s[i]*p[i].x;
            y+=s[i]*p[i].y;
            z+=s[i]*p[i].z;
        }
        point a=(point){x/down,y/down,z/down};
        point b=a-p[1];
        dd r=fabs(b*n)/s[4]/2;
        printf("%.4f %.4f %.4f %.4f\n",a.x,a.y,a.z,r);
    }
}