input: Write a function to find the union of two lists. The union of two lists is a list that contains all the elements present in either list, without duplicates. If either lists are empty, then the function should return the non-empty list as the union. The function should return a set to ensure there are no duplicates.
  assert unionList([1, 2, 3, 4, 5], [3, 4, 5, 6, 7])=={1, 2, 3, 4, 5, 6, 7}
  assert unionList([], [3, 4, 5, 6, 7])=={3, 4, 5, 6, 7}
  assert unionList([1, 2, 3, 4, 5], [])=={1, 2, 3, 4, 5}
  assert unionList([], [])==set()
  assert unionList([1, 2, 2, 3], [3, 3, 4])=={1, 2, 3, 4}
output: def unionList(list1, list2):
      if not list1:
          return set(list2)
      if not list2:
          return set(list1)
      return set(list1).union(set(list2))