code: public static int gcd(int a, int b) {
          a = Math.abs(a);
          b = Math.abs(b);
          while (a > 0) {
              int c = b % a;
              b = a;
              a = c;
          }
          return b;
      }
docs: Computes the greatest common divisor (GCD) of two integers.
  <p>
  Greatest common divisor is the largest integer that divides both numbers without remainder.
  </p>
  
  @param a
  the first integer.
  @param b
  the second integer.
  
  @return The greatest common divisor of {@code a} and {@code b}, or null,
  if both {@code a} and {@code b} are null.