Binary power

double binary_power(double a, int n){
    if(n == 0){
        return 1;
    }

    if(n % 2 == 0){
        double res = binary_power(a, n / 2);

        return res * res;
    }
    else{
        if(n > 0){
            return a * binary_power(a, n - 1);
        }
        else{
            return (1 / a) * binary_power(a, n + 1);
        }
    }
}