Python快速排序

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:
            alist[leftmark], alist[rightmark] = alist[rightmark], alist[leftmark]

    alist[first], alist[rightmark] = alist[rightmark], alist[first]

    return rightmark

alist = [54, 26, 93, 17, 77, 31, 44, 55, 20]
quickSort(alist)
print(alist)

清晰的解法：

# quick sort
def quickSort(L, low, high):
    i = low
    j = high
    if i >= j:
        return
    key = L[i]
    while i < j:
        while i < j and L[j] >= key: # 从后向前找比key小的值
            j -= 1
        L[i] = L[j] # 把比key小的值移到key在的位置
        while i < j and L[i] <= key: # 从前向后找比key大的值
            i += 1
        L[j] = L[i] # 把比key大的值移到刚找出的比key小的值的位置
    L[i] = key  # 这时的i左右分别为比key小和大的值，key移到i处
    quickSort(L, low, i-1) # 继续排前一部分
    quickSort(L, j+1, high) # 继续排后一部分
    return

测试用例

a = [4, 5, 3, 12, 43, 1, 0, 34]
quickSort(a, 0, len(a) - 1)
print(a)

结果

[0, 1, 3, 4, 5, 12, 34, 43]

最好和最坏情况

最好：key每次都能使序列均匀划分，时间复杂度为O(nlogn)
最坏：key为最大或最小数字，时间复杂度为O(n^2)