input: Write a function to determine the number of unique elements in a given integer array. If the given array is empty then return 0.
  
  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([1, 3, 5, 7, 9, 11, 13])==7
  assert num_unique_elements([])==0
  assert num_unique_elements([10])==1
output: def num_unique_elements(arr):
      if not arr:
          return 0
      return len(set(arr))