How do you find the middle of a linked list? | Interview Question

Q: How do you find the middle of 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 You are handed a long paper scroll and asked to cut it exactly in half. The catch: you can only unroll it from the start, and you have no ruler. One way is to unroll the whole thing counting the panels (say 40), roll it back up, and unroll again counting to 20. That works, but you went over the scroll one and a half times. A friend suggests a trick. Two people walk along the unrolled scroll from the start, one taking one panel per step and the other taking two. When the fast walker steps off the end,

The problem

Question presented to candidate: "Given the head of a singly linked list, return its middle node. If the list has an even number of nodes, return the second of the two middle nodes. Please do it in Python, in one pass if you can."

What a strong answer should cover:

The baseline: count the nodes, then walk half way (two passes, O(1) space), or copy into an array (O(n) space).
The one-pass fast and slow pointer loop, and why slow is in the middle when fast runs out.
Even lengths: which middle each starting position gives, and how to get the other one.
O(n) time and O(1) space, and the honest point that fast/slow does about as many pointer hops as two passes.
Python specifics: // for the index in the array version, the fast is not None and fast.next is not None order, and no len() on a hand-made node class.

Clarifying questions expected:

"For an even length, should I return the first or the second middle?"
"Should I return the node or its value, and what for an empty list?"

Code / implementation expected: Yes — a Python middle(head) with slow and fast pointers, and a note on the first-middle variant.

linked-list
two-pointers

Try it yourself

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

Stuck? Get a hint.

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

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

You are handed a long paper scroll and asked to cut it exactly in half. The catch: you can only unroll it from the start, and you have no ruler. One way is to unroll the whole thing counting the panels (say 40), roll it back up, and unroll again counting to 20. That works, but you went over the scroll one and a half times.

A friend suggests a trick. Two people walk along the unrolled scroll from the st

The solution is hidden so you can try first

11 min read · 11 sections

Python code

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

PythonFirst and second middle for every length from 0 to 6

Practice

PythonCopy to array vs two passes vs fast/slow: hops and memory

Practice

Discussion

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