input: def remove_duplicates(nums: List[int]) -> List[int]:
      """
      Remove all duplicate elements from a list and preserve the original order.
      >>> remove_duplicates([1, 3, 3, 5, 5, 7])
      [1, 3, 5, 7]
      """
output:     seen = set()
      result = []
      for num in nums:
          if num not in seen:
              seen.add(num)
              result.append(num)
      return result