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