以下是我的代碼拋出的錯(cuò)誤： File "python", line 39, in <module>  File "python", line 8, in quicksort  File "python", line 8, in quicksortTypeError: unsupported operand type(s) for -: 'NoneType' and 'int'下面是我的快速排序代碼：#!/usr/bin/python# -*- coding: utf-8 -*-def quicksort(arr, beg, end):    if beg < end:        pivot = partition(arr, beg, end)        quicksort(arr, beg, pivot - 1)        quicksort(arr, pivot + 1, end)def partition(arr, beg, end):    left = beg    right = end    loc = beg    while arr[loc] <= arr[right] and loc != right:        right = right - 1    if loc == right:        return loc    elif arr[loc] > arr[right]:        (arr[loc], arr[right]) = (arr[right], arr[loc])        loc = right    while arr[loc] >= arr[left] and loc != left:        left = left + 1    if loc == left:        return loc    elif arr[loc] < arr[left]:        (arr[loc], arr[left]) = (arr[left], arr[loc])        loc = lefty = [9,7,5,1,5]quicksort(y, 0, 4)print(y)