input: Write a function to determine the number of unique elements in an integer array. The function should return the count of unique elements. 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])==6
  assert num_unique_elements([1, 1, 1, 1])==1
  assert num_unique_elements([])==0
  assert num_unique_elements([5])==1
  assert num_unique_elements([10, 11, 12, 13, 14, 14, 14])==5
output: def num_unique_elements(arr):
      if not arr:
          return 0
      return len(set(arr))