input: Write a function to sort an array using the quicksort algorithm.
  assert quicksort([3, 6, 8, 10, 1, 2, 1]) == [1, 1, 2, 3, 6, 8, 10]
  assert quicksort([1, 4, 5, 2, 3, 6]) == [1, 2, 3, 4, 5, 6]
  assert quicksort([9, 7, 5, 11, 12, 2]) == [2, 5, 7, 9, 11, 12]
output: def quicksort(arr):
      if len(arr) <= 1:
          return arr
      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)