快速排序算法中的中间分区 Python

Question

I'm trying to implement quicksort algorithm which uses middle partitioning method. below is my code:

def middlepartition(A, p, r):
    pvi = (p + r) // 2
    pv = A[pvi]

    while p < r:
        while p < len(A) and A[p] <= pv:
            p += 1
        while A[r] > pv:
            r -= 1
        if p < r:
            A[p], A[r] = A[r], A[p]
    A[r], A[pvi] = A[pvi], A[r]

    return r


def quicksort(A, p, r):
    if p < r:
        q = middlepartition(A, p, r)
        quicksort(A, p, q - 1)
        quicksort(A, q + 1, r)


A = [0, 1, 5, 23, 0, 2, 5, 56, 79, 3, 65]
quicksort(A, 0, len(A)-1)
print(A)

but somehow the code doesn't work fine and I'm really confused. the following is the output:

[0, 1, 2, 5, 23, 0, 3, 5, 56, 65, 79]

I seriously cant find the mistake...

Answer 1

This is your mistake:

A[r], A[pvi] = A[pvi], A[r]

(or at least a mistake - there could be other issues).

This bit of code occurs in variants of QuickSort that initially move (swap) the pivot element to the end of the array and then exclude that slot from the body of the partitioning loop. Once the partitioning loop is done, the initial pivot element is moved into place at the point where the pivot index has emerged.

That's not what you're doing here, though - when control reaches the above statement the pvi index points to a random element, so the swap messes you up. Just get rid of it - the partitioning loop will have already put the pivot element in the right place at that point.