图论--生成树计数模板

bool zero(double a)
{
	return a>-eps && a<eps;
}
double Gauss()
{
	double mul,Result=1;
	int i,j,k,b[n];
	for(i=0;i<n;i++) b[i]=i;
	for(i=0;i<n;i++){
		if(zero(a[b[i]][i]))
			for(j=i+1;j<n;j++)
				if(!zero(a[b[j]][i])) { swap(b[i],b[j]); Result*=-1; break;  }
		Result*=a[b[i]][i];
		for(j=i+1;j<n;j++)
			if(!zero(a[b[j]][i])){
				mul=a[b[j]][i]/a[b[i]][i];
				for(k=i;k<n;k++)
					a[b[j]][k]-=a[b[i]][k]*mul;
			}
	}
	return Result;
}