6种排序算法的Python实现

1.冒泡排序

思路：遍历列表，每一轮每次比较相邻两项，将无序的两项交换，下一轮遍历比前一轮比较次数减1。

def bubble_sort(a_list):
    for passnum in range(len(a_list)-1, 0, -1):
        for i in range(passnum):
            if a_list[i] > a_list[i+1]:
                 a_list[i], a_list[i+1] = a_list[i+1], a_list[i]

改进：遍历期间没有交换，则认为已排序，可以停止。

def short_bubble_sort(a_list):
    exchanges = True
    passnum = len(a_list) - 1
    while passnum > 0 and exchanges:
        exchanges = False
        for i in range(passnum):
            if a_list[i] > a_list[i+1]:
                exchanges = True
                a_list[i], a_list[i+1] = a_list[i+1], a_list[i]
        passnum = passnum - 1

动画演示：

2.选择排序

思路：遍历列表，每次遍历选出最大的放在合适的位置。

def selection_sort(a_list):
    for fillslot in range(len(a_list)-1, 0, -1):
        position_max=0
        for location in range(1, fillslot+1):
            if a_list[location] > a_list[position_max]:
                position_max = location
        a_list[fillslot], a_list[position_max] = a_list[position_max], a_list[fillslot]

动画演示：

3.插入排序

思路：每一步都将待插入的数据按大小插入到已排序的数据中的合适位置。

def insertion_sort(a_list):
    for index in range(1, len(a_list)):
        cur_value = a_list[index]
        pos = index
        while pos > 0 and a_list[pos-1] > cur_value:
            a_list[pos] = a_list[pos-1]
            pos = pos - 1
        a_list[pos] = cur_value

动画演示：

4.希尔排序

思路：分解为多个子列表进行插入排序，不是连续分，而是通过增量。

def shell_sort(a_list):
    sublist_count = len(a_list)
    while sublist_count > 0:
        for start_position in range(sublist_count):
            gap_insertion_sort(a_list, start_position, sublist_count)
        sublistcount = sublistcount // 2

def gap_insertion_sort(a_list, start, gap):
    for i in range(start+gap, len(a_list), gap):
        current_value = a_list[i]
        position = i
        while position >= gap and a_list[position-gap] > current_value:
            a_list[position] = a_list[position-gap]
            position = position - gap
        a_list[position] = current_value

5.归并排序

思路：分而治之，不断拆分为一半，直至项数为0或1，然后排序合并。需要额外空间。

def merge_sort(a_list):
    if len(a_list) > 1:
        mid = len(a_list) // 2
        lefthalf = a_list[:mid]
        righthalf = a_list[mid:]
        merge_sort(lefthalf)
        merge_sort(righthalf)
        i=0
        j=0
        k=0
        while i < len(lefthalf) and j < len(righthalf):
            if lefthalf[i] < righthalf[j]:
                a_list[k] = lefthalf[i]
                i = i + 1
            else:
                a_list[k] = righthalf[j]
                j = j + 1
            k=k+1
        while i < len(lefthalf):
            a_list[k] = lefthalf[i]
            i = i + 1
            k = k + 1
        while j < len(righthalf):
            a_list[k] = righthalf[j]
            j = j + 1
            k = k + 1

动画演示：

6.快速排序

思路：与归并一样使用分而治之，不使用额外内存，特点是枢轴值。

def quick_sort(a_list):
    quick_sort_helper(a_list, 0, len(a_list)-1)

def quick_sort_helper(a_list, first, last):
    if first < last:
        splitpoint = partition(a_list, first, last)
        quick_sort_helper(a_list, first, splitpoint-1)
        quick_sort_helper(a_list, splitpoint+1, last)

def partition(a_list, first, last):
    pivotvalue = a_list[first]
    leftmark = first + 1
    rightmark = last
    done = False
    while not done:
        while leftmark <= rightmark and a_list[leftmark] <= pivotvalue:
            leftmark = leftmark + 1
        while a_list[rightmark] >= pivotvalue and rightmark >= leftmark:
            rightmark = rightmark -1
        if rightmark < leftmark:
            done = True
        else:
            a_list[leftmark], a_list[rightmark] = a_list[rightmark], a_list[leftmark]
    a_list[first], a_list[rightmark] = a_list[rightmark], a_list[first]

动画演示：

说明：

1.参考https://interactivepython.org/runestone/static/pythonds/SortSearch/toctree.html

2.动画来自https://visualgo.net/en 截图