input: 
  def min_max(arr):
      """
      Write a Python function that takes an array of numbers and returns an array with the greatest and smallest values.
  
      >>> min_max([3, 4, 2, 8, 15, 6])
      [2, 15]
      """
      min_val = min(arr)
      max_val = max(arr)
      return [min_val, max_val]
  
output:     min_val = min(arr)
      max_val = max(arr)
      return [min_val, max_val]