input: Write a function to determine the number of unique elements in a given integer array. Return the count of unique elements. If the input array is empty, return 0.
  
  assert num_unique_elements([1, 2, 3, 2, 1, 4, 7, 8, 9, 8])==7
  assert num_unique_elements([3, 4, 5, 1, 2])==5
  assert num_unique_elements([3, 5, 10, 1, 2])==5
  assert num_unique_elements([])==0
  assert num_unique_elements([4, 4, 4, 4])==1
output: def num_unique_elements(arr):
      if not arr:
          return 0
      return len(set(arr))