尝试编写快速排序的就地实现，但在返回排序数组时遇到问题

Question

I've tried writing an in-place quicksort and I'm unable to properly return the sorted array. Based on the output from the print statement, the algorithm does appear to be sorting the array but I can't seem to return the sorted array. I thought it would be sorting in place but it doesn't seem to be and I'm not sure how to correct this issue.

def quicksort(array):

    if len(array) <= 1:
        return 

    pivot = len(array) - 1
    i = 0

    while i < pivot:
        if array[i] > array[pivot]:
            if i == pivot - 1:
                array[i], array[pivot] = array[pivot], array[i]
            
            array[pivot], array[pivot-1] = array[pivot-1], array[pivot]
            pivot -= 1
            array[i], array[pivot+1] = array[pivot+1], array[i]
        else:
            i += 1
        
    print(array)
    quicksort(array[:pivot])    # sorts lower
    quicksort(array[pivot:])    # sorts upper

    return array


test = [21, 4, 1, 3, 9, 20, 25, 6, 21, 14]
print quicksort(test)

Answer 1

The problem here is that you are slicing a list , which returns a copy and not a view . Thus each of your recursive call sort a copy and not the original list. I recommend you use a numpy.array as input to perform your quicksort in-place. Indeed (basic) slicing a numpy.array returns a view . This is the python data structure that is the closest to a canonical array.

Answer 2

When you make the recursive call quicksort(array[:pivot]) , you don't pass the original list to quicksort() , but instead create a new list.

To avoid creating new lists, you can drop the slice syntax and instead pass array indices to your function:

def quicksort(array, start, length):

    if length <= 1:
        return 

    pivot = start + length - 1
    i = start

    while i < pivot:
        if array[i] > array[pivot]:
            if i == pivot - 1:
                array[i], array[pivot] = array[pivot], array[i]
            
            array[pivot], array[pivot-1] = array[pivot-1], array[pivot]
            pivot -= 1
            array[i], array[pivot+1] = array[pivot+1], array[i]
        else:
            i += 1
        
    print(array[start:start+length])
    quicksort(array, start, pivot-start)    # sorts lower
    quicksort(array, pivot, length-pivot)   # sorts upper

    return array


test = [21, 4, 1, 3, 9, 20, 25, 6, 21, 14]
print quicksort(test, 0, len(test))