input: Write a python function to count unique elements in a list.
  assert countUnique([1, 2, 3, 1, 2]) == 3
  assert countUnique([1, 1, 1, 1, 1]) == 1
  assert countUnique([1, 2, 3, 4, 5]) == 5
  assert countUnique([]) == 0
output: def countUnique(lst):
      return len(set(lst))