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.
  assert min_max([3, 4, 2, 8, 15, 6])==[2, 15]
  assert min_max([1])==[1, 1]
  assert min_max([])==[]
  assert min_max([-5, 0, 10, -10, 5])==[-10, 10]
  assert min_max([7, 7, 7, 7])==[7, 7]
output: def min_max(arr):
      if not arr:
          return []
      return [min(arr), max(arr)]