How do you move all zeroes to the end of an array in place?

Q: How do you move all zeroes to the end of an array in place?

**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 A librarian has a shelf of books with some empty gaps where books were borrowed. She wants all the books pushed to the left, in the same order, and all the gaps at the right end, so new books can be added there. She does not have a second shelf to copy onto. Her method: she keeps one hand at "the next spot where a book should go" and walks along the shelf with her eyes. Every time she sees a book, she moves it to the spot her hand marks and slides her hand one place right. Gaps she simply walks past.

The problem

Question presented to candidate: "Given a list of integers, move all the zeros to the end while keeping the order of the non-zero numbers, and do it in place, without making a copy of the list. Please write it in Python."

What a strong answer should cover:

Read/write two pointers: read scans every element, write marks the next slot for a non-zero; swap (or copy) non-zeros forward.
The invariant (non-zeros in order before write, zeros between write and read) and why it guarantees order is kept.
O(n) time, O(1) extra space; why remove plus append is O(n²) and a filtered copy is O(n) space.
The two in-place variants (swap forward versus overwrite then fill) and their write counts.
Python specifics: in place means mutating the list object, so nums = [...] inside the function does nothing for the caller, while nums[:] = ... does (but builds a temporary list); the function returns None by convention.

Clarifying questions expected:

"Must the order of non-zero values be kept?"
"Is a temporary copy allowed, or must it be O(1) extra space?"

Code / implementation expected: Yes — move_zeroes(nums) in Python with read and write indices, mutating the list and returning nothing.

arrays
two-pointers

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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

A librarian has a shelf of books with some empty gaps where books were borrowed. She wants all the books pushed to the left, in the same order, and all the gaps at the right end, so new books can be added there. She does not have a second shelf to copy onto.

Her method: she keeps one hand at "the next spot where a book should go" and walks along the shelf with her eyes. Every time she sees a book, she

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10 min read · 10 sections

Python code

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

PythonMove zeroes with read/write pointers on edge cases

Practice

PythonSwap forward vs overwrite then fill: counting writes

Practice

Discussion

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