Question

Define a function named reverse that reverses the elements in its list argument (without using the method reverse!). Try to make this function as efficient as possible, and state its computational complexity using big-O notation.

Answer 1

A reverse function in Python can efficiently reverse the elements of a list with a computational complexity of O(n) by using a while loop to swap elements from the ends to the middle.

Step-by-step explanation:

The function reverse can be defined to reverse the elements of a list without using the built-in reverse() method. The goal is to achieve this in the most efficient manner possible and express its computational complexity in big-O notation.

Python Function Definition

To define such a function in Python, one can use the following code:

def reverse(lst):
start_index = 0
end_index = len(lst) - 1
while start_index < end_index:
lst[start_index], lst[end_index] = lst[end_index], lst[start_index]
start_index += 1
end_index -= 1
return lst

This code snippet swaps the first and last elements, then moves inwards until it reaches the center of the list, effectively reversing the list. The computational complexity of this reverse function is O(n/2), which simplifies to O(n) where n represents the length of the list. This is because each element swap operation is constant time, and we are only iterating through half of the list due to the pairwise swaps.