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