How do I reverse a sublist in a list in place?

Question

I\'m supposed to create a function, which input is a list and two numbers, the function reverses the sublist which its place is indicated by the two numbers. for example this is

Answer 1

Two methods in-place and constant memory:

def reverse_swap(arr, start=None, end=None):
    """
    Swap two edge pointers until meeting in the center.
    """

    if start is None:
        start = 0
    if end is None:
        end = len(arr)

    i = start
    j = end - 1
    while i < j:
        arr[i], arr[j] = arr[j], arr[i]
        i += 1
        j -= 1

def reverse_slice(arr, start=None, end=None):
    """
    Use python slice assignment but use a generator on the right-hand-side
    instead of slice notation to prevent allocating another list.
    """

    if start is None:
        start = 0
    if end is None:
        end = len(arr)

    arr[start:end] = (arr[i] for i in range(end - 1, start - 1, -1))