input: Write a function to count the number of unique elements in a given 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, 2, 3, 4, 5])==5
  assert num_unique_elements([1, 1, 1, 1, 1])==1
  assert num_unique_elements([])==0
output: def num_unique_elements(arr):
      if not arr:
          return 0
      return len(set(arr))