public static int gcd(int first, int second) {
        if (first == 0) {
            return second;
        } else if (second == 0) {
            return first;
        } else if (first > second) {
            return gcd(first - second, second);
        } else {
            return gcd(first, second - first);
        }

    }
(**) Determine the greatest common divisor of two positive integer numbers.
java8/src/main/java/com/shekhargulati/ninetynine_problems/_02_arithmetic/P37.java