Array Reversal Techniques in DSA Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main idea behind reversing an array?

Reversing an array means changing the order of elements so the first element becomes the last, the second becomes the second last, and so on, effectively flipping the array.

beginner

How does the two-pointer technique work for array reversal?

Two pointers start at the beginning and end of the array. They swap elements and move towards the center until they meet or cross, reversing the array in place.

beginner

What is a simple Python one-liner to reverse a list?

Using slicing: reversed_list = original_list[::-1] which creates a new reversed list without modifying the original.

intermediate

Why is in-place reversal more memory efficient than creating a new reversed array?

In-place reversal swaps elements within the original array without extra space, saving memory compared to making a new reversed copy.

beginner

What is the time complexity of reversing an array using the two-pointer method?

The time complexity is O(n), where n is the number of elements, because each element is visited once during swapping.

Which method reverses an array without using extra space?

A Using slicing with [::-1]

B Creating a new reversed array

C Two-pointer swapping

D Using a for loop to append elements to a new list

Two-pointer swapping reverses the array in place without extra memory, unlike the other methods that create new arrays.

What does the Python slice notation [::-1] do?

A Duplicates the list

B Sorts the list in ascending order

C Removes the last element

D Reverses the list creating a new one

The slice [::-1] creates a new list with elements in reverse order.

In the two-pointer technique, where do the pointers start?

A One at start, one at end

B Both at the end

C Both at the start

D One in the middle, one at start

One pointer starts at the beginning and the other at the end to swap elements moving towards the center.

What is the time complexity of reversing an array of size n?

A O(1)

B O(n)

C O(n^2)

D O(log n)

Reversing requires visiting each element once, so time complexity is O(n).

Which of these is NOT a benefit of in-place array reversal?

A Creates a backup copy

B Faster than creating new array

C Saves memory

D Modifies original array

In-place reversal does not create a backup copy; it modifies the original array directly.

Explain how the two-pointer technique reverses an array step-by-step.

Think of swapping pairs from outside moving to the center.

Describe the difference between reversing an array in-place and creating a new reversed array.

Consider memory use and whether the original array changes.