What is a stable sorting algorithm? | Interview Question

Q: What is a stable sorting algorithm?

**Target Audience:** Beginners who know what sorting is and want to understand the guarantees behind it. **Difficulty:** Easy **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 exact words an interviewer expects. Every program in this doc was run with `python3 main.py`, and the output quoted in the **Verified** section is copied from that run. ## 1. Why This Even Matters — A Story First A teacher has the class list in alphabetical order. She wants the list grouped by grade, A first, but **still alphabetical inside each grade**, so she can find a name quickly. She clicks "sort by grade" in her spreadsheet. If the spreadsheet's sort is **stable**, she gets exactly what she wanted: within the A students, Ana is still before Bo, because that is the order they were in before. If the sort is **unstable**, the A students might come out as Bo, Ana, and the B students in some random-lo

The problem

Question presented to candidate: "What does it mean for a sorting algorithm to be stable? Give an example where it matters, tell me which common algorithms are stable, and whether the built-in sort in Python is."

What a strong answer should cover:

The definition: items with equal keys keep their input order; it only matters when items carry more than the key.
Why it matters: multi-key sorting in passes (least important key first), tables with several sort columns, ties in rankings, radix sort.
Stable algorithms (insertion, merge, counting, Timsort) versus usually unstable ones (selection, quicksort, heapsort).
How to make any sort stable: the original index as a tie-breaker, or a full composite key.
Python specifics: list.sort() and sorted() are guaranteed stable (Timsort), so multi-key sorts can be done in passes or with a tuple key, and reverse=True keeps equal items in their original order.

Code / implementation expected: Optional — a multi-key sort with the Python built-in, or a short demonstration of an unstable sort.

stability
sorting

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Solution

Target Audience: Beginners who know what sorting is and want to understand the guarantees behind it. Difficulty: Easy 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 exact words an interviewer expects. Every program in this doc was run with python3 main.py, and the output quoted in the Verified section is copied from that run.

1. Why This Even Matters — A Story First

A teacher has the class list in alphabetical order. She wants the list grouped by grade, A first, but still alphabetical inside each grade, so she can find a name quickly. She clicks "sort by grade" in her spreadsheet. If the spreadsheet's sort is stable, she gets exactly what she wanted: within the A students, Ana is still before Bo, because that is the order they were in before. If the

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9 min read · 12 sections

Python code

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

PythonThe built-in stable sort with ties and multiple keys

Practice

PythonInsertion sort vs selection sort: stable vs unstable

Practice

Discussion

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