具有两个嵌套循环的非递归合并排序 - 如何？

def merge_sort(arr):
    n = len(arr)
    step = 1
    while step < n:
        start = 0
        while start < n:
            mid = min(start + step - 1, n - 1)
            end = min(start + 2 * step - 1, n - 1)
            merge(arr, start, mid, end)
            start += 2 * step
        step *= 2

def merge(arr, start, mid, end):
    left = arr[start:mid+1]
    right = arr[mid+1:end+1]
    i = j = 0
    k = start
    while i < len(left) and j < len(right):
        if left[i]< right[j]:
            arr[k] = left[i]
            i += 1
        else:
            arr[k] = right[j]
            j += 1
        k += 1
    while i < len(left):
        arr[k] = left[i]
        i += 1
        k += 1
    while j < len(right):
        arr[k] = right[j]
        j += 1
        k += 1