input: Write a function that takes an array of numbers and returns an array with the greatest and smallest values. If the input array is empty, return an empty array.
  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([7, 3, 5, 1, 9])==[1, 9]
  assert min_max([4])==[4, 4]
  assert min_max([10, -5, 20, 15, 0])==[-5, 20]
  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]