input: Write a python function to flatten a list of lists into a single list.
  assert flatten_list([[1, 2, 3], [4, 5], [6, 7, 8, 9]]) == [1, 2, 3, 4, 5, 6, 7, 8, 9]
  assert flatten_list([[10, 20], [30, 40], [50]]) == [10, 20, 30, 40, 50]
  assert flatten_list([[], [1], [2, 3], [], [4, 5]]) == [1, 2, 3, 4, 5]
output: def flatten_list(list_of_lists):
      return [item for sublist in list_of_lists for item in sublist]