PythonAlgorithms/QuickSort.py at master · IKMalik/PythonAlgorithms · GitHub

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def quicksort(data):

    if len(data) < 2:
        raise ValueError("Too few items to sort ")

    else:
        quicksorthelper(data, 0, len(data)-1)


def quicksorthelper(data, lo, hi):

    if lo < hi: # if not single element
        splitpoint = partition(data, lo, hi)
        quicksorthelper(data, lo, splitpoint-1)
        quicksorthelper(data, splitpoint+1, hi)


def partition(data, lo, hi):

    pivotval = data[lo]
    leftmark = lo+1
    rightmark = hi

    complete = False
    while not complete:

        while leftmark <= rightmark and data[leftmark] <= pivotval: # why < rightmark instead of < hi
            leftmark += 1


        while rightmark >= leftmark and data[rightmark] >= pivotval: # why > leftmark mark instead of > lo
            rightmark -= 1

        if rightmark < leftmark:
            complete = True

        else:
            data[leftmark], data[rightmark] = data[rightmark], data[leftmark]

    data[lo], data[rightmark] = data[rightmark], data[lo]
    return rightmark


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