input: Write a python function to implement quicksort.
  assert quicksort([735, 422, 662, 310, 536, 721, 705, 129, 156, 799]) == [129, 156, 310, 422, 536, 662, 705, 721, 735, 799]
  assert quicksort([3, 6, 8, 10, 1, 2, 1]) == [1, 1, 2, 3, 6, 8, 10]
  assert quicksort([1]) == [1]
  assert quicksort([]) == []
output: def quicksort(arr):
      if len(arr) <= 1:
          return arr
      else:
          pivot = arr[len(arr) // 2]
          left = [x for x in arr if x < pivot]
          middle = [x for x in arr if x == pivot]
          right = [x for x in arr if x > pivot]
          return quicksort(left) + middle + quicksort(right)