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