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