How do you check if a linked list is a palindrome?

Q: How do you check if a linked list is a palindrome?

**Target Audience:** Beginners preparing for coding interviews — you should know what a linked list node is; everything else is explained **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 Imagine a conga line of people, each holding the shoulders of the person in front. Every person wears a number on their back. You want to know whether the numbers read the same from the front of the line as from the back. With a row of chairs you would simply look at both ends. But in a conga line you can only start at the front and walk forward, one shoulder at a time; nobody is holding anyone from behind. One easy trick is to write every number on a notepad as you

The problem

Question presented to candidate: "Given the head of a singly linked list, return True if its values read the same forwards and backwards. First tell me the simple way, then do it in O(1) extra space. Please write it in Python."

What a strong answer should cover:

The simple version: copy values into a Python list and compare (vals == vals[::-1]), O(n) time and O(n) space.
The O(1)-space version: find the middle with fast and slow pointers, reverse the second half in place, compare value by value, then reverse it back.
Correct middle for odd and even lengths, and a comparison loop driven by the shorter (second) half.
Restoring the list before returning, even when the answer is False.
Python specifics: the tuple-assignment reversal head.next, prev, head = prev, head, head.next (order matters), compare val with ==, and the recursive version hits the default recursion limit of 1000 on long lists.

Clarifying questions expected:

"Is O(n) extra space allowed? May I modify the list temporarily?"
"Is an empty list a palindrome?"

Code / implementation expected: Yes — is_palindrome(head) in Python, ideally the O(1)-space version.

linked-list
two-pointers

Try it yourself

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

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A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Beginners preparing for coding interviews — you should know what a linked list node is; everything else is explained 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

Imagine a conga line of people, each holding the shoulders of the person in front. Every person wears a number on their back. You want to know whether the numbers read the same from the front of the line as from the back. With a row of chairs you would simply look at both ends. But in a conga line you can only start at the front and walk forward, one shoulder at a time

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11 min read · 10 sections

Python code

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

PythonReverse-the-second-half check, with the list printed afterwards

Practice

PythonEvery 0/1 list up to 8 nodes, against the array version

Practice

Discussion

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