input: 
  def common_elements(list1: List[int], list2: List[int]) -> List[int]:
      """ Find the common elements in two given lists of integers.
      >>> common_elements([3, 9, 15, 21, 27], [21, 7, 13, 29, 31])
      [21]
      >>> common_elements([1, 2, 3], [3, 4, 5])
      [3]
      """
  
output:     common = []
      for num in list1:
          if num in list2 and num not in common:
              common.append(num)
      return common