input: from typing import List
  
  def deleteDuplicates(list: List[int]) -> List[int]:
      """
      Deletes all duplicates in a given list of numbers.
      
      >>> deleteDuplicates([1, 2, 3, 4, 5, 2, 1])
      [1, 2, 3, 4, 5]
      >>> deleteDuplicates([1, 1, 2, 2, 3, 3, 4, 4, 5, 5])
      [1, 2, 3, 4, 5]
      >>> deleteDuplicates([10, 20, 30, 20, 10])
      [10, 20, 30]
      """
output:     seen = set()
      result = []
      for num in list:
          if num not in seen:
              seen.add(num)
              result.append(num)
      return result