将分而治之的递归算法转换为迭代版本

def quick_sort(arr):
    if len(arr) <= 1:
        return arr
    pivot = arr[len(arr) // 2]
    left = [x for x in arr if x < pivot]
    middle = [x for x in arr if x == pivot]
    right = [x for x in arr if x > pivot]
    return quick_sort(left) + middle + quick_sort(right)

def quick_sort_iterative(arr):
    stack = [(0, len(arr) - 1)]
    while stack:
        start, end = stack.pop()
        if start >= end:
            continue
        pivot = arr[start]
        left = start + 1
        right = end
        while left <= right:
            while left <= right and arr[left] < pivot:
                left += 1
            while left <= right and arr[right] >= pivot:
                right -= 1
            if left <= right:
                arr[left], arr[right] = arr[right], arr[left]
        arr[start], arr[right] = arr[right], arr[start]
        stack.append((start, right - 1))
        stack.append((right + 1, end))
    return arr