input: Write a function that takes an array of numbers and returns an array with the greatest and smallest values.
  
  def min_max(arr):
    if not arr:
      return []
    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([1, 2, 3, 4, 5])==[1, 5]
  assert min_max([7, 3, 9, 10, 2])==[2, 10]
  assert min_max([])==[]
output: def min_max(arr):
    if not arr:
      return []
    min_val = min(arr)
    max_val = max(arr)
    return [min_val, max_val]