input: Write a python 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([10,7,8,9,1,5]) == [1,5,7,8,9,10]
  assert quicksort([1,4,3,5,6,2]) == [1,2,3,4,5,6]
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)