How do you reverse a linked list? | Interview Question

Q: How do you reverse a linked list?

**Target Audience:** Beginners preparing for coding interviews — no prior algorithms knowledge assumed **Difficulty:** Easy **Language:** Python 3 > **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact word an interviewer expects. All code is Python, and every output block in the **Verified** section was copied from a real run of the examples on this page. ## 1. Why This Even Matters — A Story First Picture a treasure hunt. Each clue card says where the next clue is hidden: card 1 says "look under the mat", the mat card says "look in the fridge", and so on until the last card says "the end". You only ever hold the first card. Now the organiser asks you to run the hunt **backwards** for the next group, starting from the prize. You cannot photocopy the cards and you have no table to spread them out on. What you can do is walk the route once, and at every stop rewrite the card so it points to the p

The problem

Question presented to candidate: "Given the head of a singly linked list, reverse it and return the new head. Write it in Python, and tell me how much extra memory it uses."

What a strong answer should cover:

The idea: walk the list once and point every node's next at the node before it, so the old tail becomes the head.
Three pointers (prev, curr, next) and why the next node must be saved before the arrow is flipped.
O(n) time and O(1) extra space for the iterative version; the recursive version is O(n) time but O(n) call-stack space.
Edge cases: empty list, one node, two nodes, and returning the new head rather than the old one.
Python specifics: a small ListNode class, is None checks, why the order of a one-line tuple assignment matters, and why recursion fails past CPython's default limit of about 1,000 frames.

Clarifying questions expected:

"Singly or doubly linked, and may I change the nodes in place or must I build a new list?"
"How long can the list be? That decides whether recursion is safe."

Code / implementation expected: Yes — a Python reverse_list(head) with prev / curr / nxt, ideally plus the recursive version and its stack cost.

linked-list

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

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Solution

Target Audience: Beginners preparing for coding interviews — no prior algorithms knowledge assumed Difficulty: Easy Language: Python 3

How to read this doc: Every idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact word an interviewer expects. All code is Python, and every output block in the Verified section was copied from a real run of the examples on this page.

1. Why This Even Matters — A Story First

Picture a treasure hunt. Each clue card says where the next clue is hidden: card 1 says "look under the mat", the mat card says "look in the fridge", and so on until the last card says "the end". You only ever hold the first card.

Now the organiser asks you to run the hunt backwards for the next group, starting from the prize. You cannot photocopy the cards and you have no table to spread them out

The solution is hidden so you can try first

14 min read · 13 sections

Python code

Edit the code and run it here. Each printed line with an expected value becomes a test.

PythonIterative reversal with three pointers

Practice

PythonRecursive reversal and the call depth it needs

Practice

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