Question

Python - Recursive to non-recursive quick sort. What I have at the moment is a recursive quick sort, I need to make it non-recursive, any help is appreciated!

def quickSort(list):
    quickSortHelper(list, 0, len(list) - 1)

def quickSortHelper(list, first, last):
    if last > first:
        pivotIndex = partition(list, first, last)
        quickSortHelper(list, first, pivotIndex - 1)
        quickSortHelper(list, pivotIndex + 1, last)

# Partition list[first..last]
def partition(list, first, last):
    pivot = list[first] # Choose the first element as the pivot
    low = first + 1 # Index for forward search
    high = last # Index for backward search

    while high > low:
        # Search forward from left
        while low <= high and list[low] <= pivot:
            low += 1

        # Search backward from right
        while low <= high and list[high] > pivot:
            high -= 1

        # Swap two elements in the list
        if high > low:
            list[high], list[low] = list[low], list[low]

    while high > first and list[high] >= pivot:
        high -= 1

    # Swap pivot with list[high]
    if pivot > list[high]:
        list[first] = list[high]
        list[high] = pivot
        return high
    else:
        return first

# A test function
def main():
    list = [2, 3, 2, 5, 6, 1, -2, 3, 14, 12]
    quickSort(list)
    for v in list:
        print(str(v) + " ", end = "")

main()

Answer 1

Python code is given below: (modified code segment is highlighted)

def quickSort(list):
    quickSortHelper(list, 0, len(list) - 1)

def quickSortHelper(list, first, last):
    if last > first:
        pivotIndex = partition(list, first, last)
        quickSortHelper(list, first, pivotIndex - 1)
        quickSortHelper(list, pivotIndex + 1, last)

# Partition list[first..last]
def partition(list, first, last):
    pivot = list[first] # Choose the first element as the pivot
    low = first + 1 # Index for forward search
    high = last # Index for backward search

    while high > low:
        # Search forward from left
        while low <= high and list[low] <= pivot:
            low += 1

        # Search backward from right
        while low <= high and list[high] > pivot:
            high -= 1

        # Swap two elements in the list
        if high > low:
            temp = list[high]
            list[high] = list[low]
            list[low] = temp

    while high > first and list[high] > pivot:
        high -= 1

    # Swap pivot with list[high]
    if pivot > list[high]:
        list[first] = list[high]
        list[high] = pivot
        return high
    else:
        return first


def quickSortIterative(list):
    l = 0 # set the low and high
    h = len(list) - 1
    size = h - l + 1  
    st = [0] * (size)  # initializing the stack
    top = -1 # it represents the empty stack
    top = top + 1
    st[top] = l # inserting low and high in stack
    top = top + 1
    st[top] = h
    while top >= 0: # run while loop until the stack becomes empty
        h = st[top] # popping the low and high
        top = top - 1
        l = st[top]
        top = top - 1
        p = partition(list, l, h) # partition 
        if p - 1 > l: # again push the range low to pivot-1
            top = top + 1
            st[top] = l
            top = top + 1
            st[top] = p - 1
        if p + 1 < h:  # push the range pivot + 1 to high
            top = top + 1
            st[top] = p + 1
            top = top + 1
            st[top] = h

# A test function
def main():
    list = [2, 3, 2, 5, 6, 1, -2, 3, 14, 12]
    quickSortIterative(list)
    for v in list:
        print(str(v) + " ", end = "")

main()

Sample Output:
-2 1 2 2 3 3 5 6 12 14

Screenshot of the code is given below:

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