input: Write a function to determine the number of unique elements in a given integer array.
  
  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, 2, 3, 4, 5])==5
  assert num_unique_elements([])==0
  assert num_unique_elements([10, 20, 30, 20, 10, 5, 5])==4
output: def num_unique_elements(arr):
      return len(set(arr))