Reverse a Doubly Linked List in DSA Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a doubly linked list?

A doubly linked list is a chain of nodes where each node has three parts: data, a pointer to the next node, and a pointer to the previous node. This allows moving forward and backward through the list.

beginner

What does reversing a doubly linked list mean?

Reversing a doubly linked list means changing the direction of the list so that the last node becomes the first, and the first becomes the last. The next and previous pointers of each node are swapped.

intermediate

In reversing a doubly linked list, what pointers do we swap in each node?

We swap the 'next' pointer with the 'prev' pointer in each node. This changes the direction of the links between nodes.

intermediate

Why do we update the head pointer after reversing a doubly linked list?

Because after reversing, the original tail becomes the new head. We must update the head pointer to point to this new first node to correctly represent the reversed list.

beginner

What is the time complexity of reversing a doubly linked list?

The time complexity is O(n), where n is the number of nodes. We visit each node once to swap pointers.

What pointers are swapped in each node when reversing a doubly linked list?

A Next and previous pointers

B Data and next pointers

C Only next pointers

D Only previous pointers

Reversing swaps the next and previous pointers in each node to reverse the direction.

After reversing a doubly linked list, what does the head pointer point to?

A Original tail node

B Original head node

C Null

D Middle node

The original tail becomes the new head after reversing.

What is the time complexity of reversing a doubly linked list with n nodes?

A O(n^2)

B O(n)

C O(1)

D O(log n)

Each node is visited once to swap pointers, so the time is linear.

Which of these is NOT true about a doubly linked list?

A Each node has pointers to next and previous nodes

B It allows traversal in both directions

C It can be reversed by swapping pointers

D It stores data only in the head node

Data is stored in every node, not only the head.

What happens if you forget to update the head pointer after reversing a doubly linked list?

A Nothing changes

B The list will reverse automatically

C The list will appear empty or incorrect

D The tail pointer updates instead

Without updating head, the list start is wrong, so traversal fails.

Explain step-by-step how to reverse a doubly linked list.

Think about how pointers change direction and how to track the new head.

Describe why reversing a doubly linked list is easier than reversing a singly linked list.

Compare pointer availability in both list types.