input: Write a python function to merge two lists into a dictionary.
  assert list_to_dict([1, 2, 3], ['a', 'b', 'c']) == {1: 'a', 2: 'b', 3: 'c'}
  assert list_to_dict([4, 5, 6], ['d', 'e', 'f']) == {4: 'd', 5: 'e', 6: 'f'}
  assert list_to_dict([1, 2], ['x', 'y', 'z']) == None
output: def list_to_dict(keys, values):
      if len(keys) != len(values):
          return None
      return dict(zip(keys, values))