input: Write a function to count the number of unique elements in a given integer array.
  
  
  def num_unique_elements(arr):
      return len(set(arr))
  
  assert num_unique_elements([1, 2, 3, 2, 1, 4, 7, 8, 9, 8]) == 7
  assert num_unique_elements([1, 1, 1, 1, 1]) == 1
  assert num_unique_elements([]) == 0
  assert num_unique_elements([5, 6, 7, 8, 9]) == 5
  
output: def num_unique_elements(arr):
      return len(set(arr))