In the solution, each element is inserted into the temp_list array with temp_list.insert(0, arr[i]). According to this Python wiki, List insert has O(n) time complexity, so I used a deque, for which appendleft has O(1) time complexity, instead. Should the solution be changed to use a deque as well? The empty solution imports deque, so it seems to me that it might have been the intention to use it.