Hello Algo

What it is

Hello Algo is a free, open source book on data structures and algorithms, written by krahets and many contributors in a GitHub repository. Its English README calls it a tutorial “with animated illustrations and code you can run with one click”. The book is the product here. You read it on its website, as a PDF or EPUB, or in your own editor, and Mindket’s setup notes say how to do each. The repository holds the book in Simplified and Traditional Chinese, English, Japanese, Russian and Korean.

1. Pick the book in your language
2. Read a chapter, animations first
3. Find the file named above its code block
4. Run it in your own language environment
5. Type it out yourself and change it

Who it suits

Beginners in data structures and algorithms who can already read and write simple code in one language. The About page also aims it at readers who have solved some practice problems but only half understand the theory behind them. Readers who know the common problem types can use it to review, with the repository’s code as an “algorithm dictionary”.

Skip it if you want practice problems more than explanations. The preface says this book mainly covers the fundamentals, and it points to the author’s LeetCode-Book repository for problem lists.

Where to use it

The book works in a browser, in an e-book reader, in your editor and in Docker. The maker documents no coding agent, so these notes have none.

• The website: the Simplified Chinese, Traditional Chinese, English, Japanese, Russian and Korean editions, each at its own address
• PDF and EPUB files: on the GitHub releases page, for Simplified Chinese, Traditional Chinese and English, with a separate file for each code language
• Your editor: the appendix recommends VS Code and gives setup steps for Python, C and C++, Java, C#, Go, Swift, JavaScript, TypeScript, Dart and Rust
• In the page: the “Visualize execution” button under a Python code block shows it running through pythontutor
• Docker: files at the repository’s root build the whole website on your machine, and each edition’s code folder has files that build a container for its code. The notes describe the English one

How to set it up

Get it saves a zip of Mindket’s setup notes in your downloads folder, with a note for each way of reading or running the book. Reading online needs nothing at all.

1. Open the book in English or Chinese. The English edition is at https://www.hello-algo.com/en/ , the Simplified Chinese one at https://www.hello-algo.com/ .
2. Read “How to use this book” first. That preface page recommends reading it in full.
3. Set up your language to run code. The appendix gives the steps, and the notes list them.
4. Clone the repository, or download the ZIP.
5. Open a source file from the en/codes folder. A code block with a file name above it tells you which one.

git clone https://github.com/krahets/hello-algo.git

To build your own copy of the site, run this in the repository’s root folder. The English edition is then at http://localhost:8000/en/ .

docker-compose up -d

This line removes the copy again.

docker-compose down

How to use it

Start with the preface page, then go chapter by chapter or jump to the topic you need. The preface page lists the book’s writing conventions.

When a passage has an animated illustration, the preface says to look at the illustration first and use the text to support it. It also recommends typing the code out yourself if you have time, and at least reading and running all of it if you do not.

Every section of the web edition has a comments area at the bottom. The preface says the author and fellow contributors do their best to answer there, usually within two days, and it asks readers to answer each other too.

Example

You want to understand quick sort. You open section 11.5 on the website. It shows the partition step by step in pictures, where a pivot is chosen, two pointers walk in from the ends and swap elements until they meet. Then you clone the repository and open the Python file for that section in VS Code:

en/codes/python/chapter_sorting/quick_sort.py

Mindket copied that one file into an empty folder and ran it with Python 3.14 on 3 November 2026. It needs no other file. The file holds three versions of quick sort and a driver that sorts the same six numbers with each. It printed:

After quick sort, nums = [0, 1, 2, 3, 4, 5]
After quick sort (median pivot optimization), nums1 = [0, 1, 2, 3, 4, 5]
After quick sort (recursion depth optimization), nums2 = [0, 1, 2, 3, 4, 5]

The page explains the second and third versions. Picking the median of three elements as the pivot makes a slow run less likely, and recursing only on the shorter side keeps the recursion shallow. Change the numbers in the driver at the bottom and run the file again.

For newcomers and experts

If you are new. The foreword gives everyday pictures. Winter clothes behave like a stack, because what goes on first comes off last. A tube of shuttlecocks is like a queue, and a dictionary resembles a hash table. Chapter 2 teaches you to compare algorithms by how much time and space they use. If setup feels heavy, read the whole book online first. Nothing needs installing for that.

For experts. The repository holds six editions side by side. The Simplified Chinese book sits at the root, and each of zh-hant, en, ja, ru and ko has its own docs, its own codes folder and its own mkdocs.yml. The English contributing guide treats the Chinese source as the authority for meaning.

The Markdown source of a page on GitHub does not show its code. The quick sort source holds only placeholder lines such as [file]{quick_sort}-[class]{quick_sort}-[func]{partition} where the code is drawn on the website. Read the code in the codes folder.

The English Python folder has one folder per chapter, a modules folder and test_all.py. That script finds every chapter_*/*.py file, runs each one, and raises an error if any fails. The Go folder has a pkg folder and a go.mod beside its chapters. Chapter exercise pages are generated, and the English guide says wording changes belong in the companion workspace repository.

Good to know

The notes are Mindket’s own, read from the repository on 3 November 2026. They are not made or endorsed by krahets, and they hold none of the book’s pictures or code.

• License: CC BY-NC-SA 4.0 for the texts, code, images, photos and videos, says the README. The LICENSE file is the Creative Commons legal text. GitHub names the license “Other”. The name has NonCommercial and ShareAlike in it, so read LICENSE before you reuse anything in something you sell
• Reaching out: the website loads MathJax from cdnjs.cloudflare.com, and “Visualize execution” works through pythontutor.com. The README’s pictures load from img.shields.io, readme-typing-svg.demolab.com and contrib.rocks.
• The site was busy: on 3 November 2026 hello-algo.com answered Mindket with HTTP 429, too many requests, so the notes read the pages from the repository. Mindket did not read the printed edition’s page
• Pages disagree: the README says the book supports 12 code languages, its badges show 13, the code folder also holds zig, and the appendix gives setup steps for 11
• PDF and EPUB: only release 1.3.0 has EPUB or English files, and none of its files are in Japanese, Russian or Korean
• Docker: Mindket did not run either build. The code container’s file rewrites its apt sources to Aliyun’s mirrors
• Removal: delete the folder or the file. The website copy comes down with docker-compose down