input: Write a function to count the number of unique elements in a given integer array.
  
  assert num_unique_elements([1, 2, 3, 4, 1, 2, 5, 6, 7, 6]) == 7
  assert num_unique_elements([0, 0, 0, 0]) == 1
  assert num_unique_elements([]) == 0
  assert num_unique_elements([-1, -2, -1, -3, -4, -5, -6, -5, -4]) == 6
  assert num_unique_elements([10, 20, 30, 40, 50]) == 5
output: def num_unique_elements(arr):
      return len(set(arr))