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([1, 2, 3])==[1, 3]
  assert min_max([5])==[5, 5]
  assert min_max([])==[]
  assert min_max([10, -1, 7, 9]) == [-1, 10]
output: def min_max(arr):
      if not arr:
          return []
      min_val = min(arr)
      max_val = max(arr)
      return [min_val, max_val]