How do you rotate an array by k positions?

Q: How do you rotate an array by k positions?

**Target Audience:** Beginners preparing for coding interviews — you only need Python lists, loops and the `%` operator. **Difficulty:** Medium **Language:** Python 3 > **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the word an interviewer expects. Every program here was run with `python3`, and the output shown in the **Verified** section is copied from that run. ## 1. Why This Even Matters — A Story First Seven friends sit around a round table, each in a numbered chair. The host says: "everybody move three chairs to the right." Nobody leaves the table and nobody new arrives; everyone just ends up three seats further on, and the three people at the "end" wrap around to the first chairs. That is exactly what rotating an array does. The array is a row of chairs, and "rotate right by k" means every value moves k positions to the right, with the last k values wrapping round to the front. Two things fall

The problem

Question presented to candidate: "Given a Python list nums and a non-negative integer k, rotate the list to the right by k steps, in place. For example [1, 2, 3, 4, 5, 6, 7] with k = 3 becomes [5, 6, 7, 1, 2, 3, 4]. Can you do it with O(1) extra space?"

What a strong answer should cover:

Reduce k %= n first (k can exceed n) and guard the empty list.
The brute force (rotate by one, k times, O(n·k)) and the extra-array version (O(n) space), then the three-reversal algorithm: O(n) time, O(1) space.
Why the reversals work: A B → rev(B) rev(A) → B A.
Optionally cyclic replacements, and why they may need gcd(n, k) separate cycles.
Python specifics: nums[:] = nums[-k:] + nums[:-k] mutates the caller's list but uses O(n) extra space, plain nums = ... only rebinds a local name, collections.deque.rotate(k) exists, and Python's % is never negative for a positive n.

Clarifying questions expected:

"Right or left rotation, and can k be larger than n or negative?"
"Must it be in place, or may I return a new list?"

Code / implementation expected: Yes — a Python function that rotates nums in place with three reversals.

arrays
rotation

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 — you only need Python lists, loops and the % operator. Difficulty: Medium Language: Python 3

How to read this doc: Each idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the word an interviewer expects. Every program here was run with python3, and the output shown in the Verified section is copied from that run.

1. Why This Even Matters — A Story First

Seven friends sit around a round table, each in a numbered chair. The host says: "everybody move three chairs to the right." Nobody leaves the table and nobody new arrives; everyone just ends up three seats further on, and the three people at the "end" wrap around to the first chairs.

That is exactly what rotating an array does. The array is a row of chairs, and "rotate right by k" means every value

The solution is hidden so you can try first

13 min read · 11 sections

Python code

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

PythonRotate right by k with three reversals (k > n, k = 0, empty array)

Practice

PythonBrute force vs extra array vs reversal vs cyclic replacements, counting writes