input: def common_elements_in_lists(list1: List[int], list2: List[int]) -> List[int]:
      """ Find the common elements in two given lists.
      >>> common_elements_in_lists([3, 9, 15, 21, 27], [21, 7, 13, 29, 31])
      [21]
      >>> common_elements_in_lists([1, 2, 3], [2, 3, 4])
      [2, 3]
      """
output:     set1 = set(list1)
      set2 = set(list2)
      return list(set1 & set2)