input: Write a function to count the number of unique elements in an integer array. The function should take a list of integers as input and return an integer representing the number of unique elements in the list.
  assert num_unique_elements([1, 2, 3, 2, 1, 4, 7, 8, 9, 8])==6
output: def num_unique_elements(arr):
      return len(set(arr))