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]
  
  assert min_max([3, 4, 2, 8, 15, 6])==[2, 15]
  assert min_max([-1, 0, 1, -3, 5])==[-3, 5]
  assert min_max([7, 7, 7, 7])==[7, 7]
  assert min_max([])==[]
  assert min_max([42])==[42, 42]
output: def min_max(arr):
    if not arr:
      return []
    min_val = min(arr)
    max_val = max(arr)
    return [min_val, max_val]