input: Write a function to remove all the duplicates from a linked list in Python. If the linked list is empty, the function should return an empty linked list.
  assert remove_duplicates([7, 14, 28, 28, 28, 35, 49])==[7, 14, 28, 35, 49]
  assert remove_duplicates([1, 2, 2, 3, 4, 4, 5])==[1, 2, 3, 4, 5]
  assert remove_duplicates([11, 11, 11, 11])==[11]
  assert remove_duplicates([])==[]
output: def remove_duplicates(lst):
      if not lst:
          return []
      seen = set()
      result = []
      for item in lst:
          if item not in seen:
              seen.add(item)
              result.append(item)
      return result