input: Write a python function to find the harmonic series sum up to n-1.
  assert harmonic_series(7) == 2.5928571428571425
  assert harmonic_series(4) == 2.083333333333333
  assert harmonic_series(19) == 3.547739657143682
output: def harmonic_series(n):
      return sum(1 / i for i in range(1, n))