input: Write a function to count the number of unique elements in an integer array.
  
  def num_unique_elements(arr):
      unique_arr = set(arr)
      return len(unique_arr)
  
  assert num_unique_elements([1, 2, 3, 2, 1, 4, 7, 8, 9, 8])==6
output: def num_unique_elements(arr):
      unique_arr = set(arr)
      return len(unique_arr)