input: from typing import List
  
  def union_of_lists(list1: List[int], list2: List[int]) -> List[int]:
      """ Find the union of two lists of integers.
      >>> union_of_lists([1, 2, 3, 4, 5], [3, 4, 5, 6, 7])
      {1, 2, 3, 4, 5, 6, 7}
      >>> union_of_lists([], [1, 2, 3])
      {1, 2, 3}
      """
      return set(list1 + list2)
output:     return set(list1 + list2)