input: Write a function named `num_unique_elements` that takes an integer array as input and returns the number of unique elements in the array. If the input array is empty, the function should 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
  assert num_unique_elements([-1, -2, -3, -1, -2])==3
output: def num_unique_elements(arr):
      return len(set(arr))