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]
  
  
  ### Test Cases:
  assert min_max([3, 4, 2, 8, 15, 6]) == [2, 15]
  assert min_max([10, 0, -5, 17, 3]) == [-5, 17]
  assert min_max([]) == []
  assert min_max([7]) == [7, 7]
output: def min_max(arr):
    if not arr:
      return []
    min_val = min(arr)
    max_val = max(arr)
    return [min_val, max_val]