input: Write a function to return the number of unique elements in the 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([5, 5, 5, 5, 5])==1
  assert num_unique_elements([10, 20, 30, 40, 50])==5
  assert num_unique_elements([])==0
output: def num_unique_elements(arr):
      return len(set(arr))