25_快速排序.py

# -*- coding: utf-8 -*-
# @Author : yxn
# @Date : 2022/2/3 22:35 
# @IDE : PyCharm(2021.3.1) Python3.98
def quickSort(alist):
    quickSortHelper(alist, 0, len(alist - 1))


def quickSortHelper(alist, first, last):
    if first < last:
        splitpoint = partition(alist, first, last)

        quickSortHelper(alist, first, splitpoint - 1)
        quickSortHelper(alist, splitpoint + 1, last)


def partition(alist, first, last):
    pivotvalue = alist[first]

    leftmark = first + 1
    rightmark = last

    done = False
    while not done:
        while leftmark <= rightmark and \
                alist[leftmark] <= pivotvalue:
            leftmark += 1
        while alist[rightmark] >= pivotvalue and \
                rightmark >= leftmark:
            rightmark -= 1

        if rightmark < leftmark:
            done = True

        else:
            temp = alist[leftmark]
            alist[leftmark] = alist[rightmark]
            alist[rightmark] = temp

    temp = alist[first]
    alist[first] = alist[rightmark]
    alist[rightmark] = temp

    return alist


def quick_sort(alist, start, end):
    """快速排序来源 https://blog.csdn.net/weixin_43250623/article/details/88931925"""
    if start >= end:  # 递归的退出条件
        return
    mid = alist[start]  # 设定起始的基准元素
    low = start  # low为序列左边在开始位置的由左向右移动的游标
    high = end  # high为序列右边末尾位置的由右向左移动的游标
    while low < high:
        # 如果low与high未重合，high(右边)指向的元素大于等于基准元素，则high向左移动
        while low < high and alist[high] >= mid:
            high -= 1
        alist[low] = alist[high]  # 走到此位置时high指向一个比基准元素小的元素,将high指向的元素放到low的位置上,此时high指向的位置空着,接下来移动low找到符合条件的元素放在此处
        # 如果low与high未重合，low指向的元素比基准元素小，则low向右移动
        while low < high and alist[low] < mid:
            low += 1
        alist[high] = alist[low]  # 此时low指向一个比基准元素大的元素,将low指向的元素放到high空着的位置上,此时low指向的位置空着,之后进行下一次循环,将high找到符合条件的元素填到此处

    # 退出循环后，low与high重合，此时所指位置为基准元素的正确位置,左边的元素都比基准元素小,右边的元素都比基准元素大
    alist[low] = mid  # 将基准元素放到该位置,
    # 对基准元素左边的子序列进行快速排序
    quick_sort(alist, start, low - 1)  # start :0  low -1 原基准元素靠左边一位
    # 对基准元素右边的子序列进行快速排序
    quick_sort(alist, low + 1, end)  # low+1 : 原基准元素靠右一位  end: 最后


if __name__ == '__main__':
    # testlist = [1, 100, 32, 8, 17, 19, 42, 13, 4]
    # quickSort(testlist)
    # print(testlist)

    alist = [54, 26, 93, 17, 77, 31, 44, 55, 20]
    quick_sort(alist, 0, len(alist) - 1)
    print(alist)