【计算几何】两圆交点

vector<Pair> calc(double x1 , double y1 , double r1 , double x2 , double y2 , double r2){
	 x1-=x2,y1-=y2;
	 double S=x1*x1+y1*y1,a=(S+r2*r2-r1*r1)/2,D=S*r2*r2-a*a;
	 if(D<0) return {};
	 double A1=a*x1,B1=y1*sqrt(D);
	 double A2=a*y1,B2=x1*sqrt(D);
	 return {{(A1+B1)/S+x2,(A2-B2)/S+y2},{(A1-B1)/S+x2,(A2+B2)/S+y2}};
}