input: from typing import List
  
  def num_unique_elements(arr: List[int]) -> int:
      """ Given a list of integers, returns the number of unique elements in the list.
      >>> num_unique_elements([1, 2, 3, 2, 1, 4, 7, 8, 9, 8])
      7
      """
output:     unique_elements = set(arr)
      return len(unique_elements)