code: public int[] multiply(int[] p, int[] q) {
      int len = p.length + q.length - 1;
      int[] result = new int[len];
      for (int i = 0; i < len; i++) {
        result[i] = 0;
      }
      for (int i = 0; i < p.length; i++) {
        for (int j = 0; j < q.length; j++) {
          result[i + j] = add(result[i + j], multiply(p[i], q[j]));
        }
      }
      return result;
    }
docs: Compute the multiplication of two polynomials. The index in the
  array corresponds to the power of the entry. For example p[0] is the
  constant term of the polynomial p.
  @param p input polynomial
  @param q input polynomial
  @return polynomial represents p*q