input: Write a function to return the number of unique elements in an integer array. If the array is empty, return 0.
  def num_unique_elements(arr):
      if not arr:
          return 0
      unique_arr = set(arr)
      return len(unique_arr)
  
  assert num_unique_elements([1, 2, 3, 2, 1, 4, 7, 8, 9, 8])==6
  assert num_unique_elements([1, 1, 1, 1])==1
  assert num_unique_elements([1, 2, 3, 4])==4
  assert num_unique_elements([])==0
  assert num_unique_elements([5])==1
output: def num_unique_elements(arr):
      if not arr:
          return 0
      unique_arr = set(arr)
      return len(unique_arr)