input: def num_unique_elements(arr: List[int]) -> int:
      """
      Count the number of unique elements in the given integer array.
      
      >>> num_unique_elements([1, 2, 3, 2, 1, 4, 7, 8, 9, 8])
      7
      >>> num_unique_elements([])
      0
      >>> num_unique_elements([5, 5, 5, 5])
      1
      """
output:     unique_elements = set(arr)
      return len(unique_elements)