input: Write a function to count the number of unique elements in an integer array and return the count. If the array is empty, return 0.
  def num_unique_elements(arr):
      if not arr:
          return 0
      return len(set(arr))
  
  assert num_unique_elements([1, 2, 3, 2, 1, 4, 7, 8, 9, 8]) == 7
  assert num_unique_elements([5, 5, 5, 5, 5]) == 1
  assert num_unique_elements([]) == 0
  assert num_unique_elements([1, 2, 3, 4, 5]) == 5
output: def num_unique_elements(arr):
      if not arr:
          return 0
      return len(set(arr))