input: 
  from typing import List
  
  def num_unique_elements(arr: List[int]) -> int:
      """ Given an integer array, return the number of unique elements in the array.
      >>> num_unique_elements([1, 2, 3, 2, 1, 4, 7, 8, 9, 8])
      7
      """
      unique_arr = set(arr)
      return len(unique_arr)
  
output:     unique_arr = set(arr)
      return len(unique_arr)