Why you can trust this course

What this is

A free, personal course that takes you from “I freeze in DSA rounds” to calm and interview-ready, in Java. No ads, no accounts, no paywall. It is not affiliated with LeetCode or with any company named on the site.

How it earns your trust

• Every line of Java you see comes from a real .java file: 439 files, compiled with javac --release 17 and run against their tests (edge cases included) before the site is built. Nothing is hand-typed into a page.
• Every one of the 302 simulations is a pure function of its input. Tests run each one, and check that every Java line it highlights exists in the code shown next to it.
• Every page's content is checked against a schema: each of the 343 problems has at least two approaches, each with its time and space cost and the reason for it.
• Where a problem has a judge, it is proven at build time: the reference solution must pass every test and the starter code must compile and fail. The hidden tests are only hidden from view, not secret; they run in your browser.
• Problem statements are in our own words. The official statement and judge stay on LeetCode, linked from every problem.
• Company tags mean “frequently reported” in public interview reports and lists. They are not official question banks, and they can go out of date.

Privacy

No tracking, no cookies, no accounts. Your progress (solved problems, quiz answers, drafts of your code, 3D settings) is saved only in this browser's localStorage. To move it to another browser or device, export it as a file on my progress and import it there. Fonts are served from this site. When you open the code editor on a problem, the Java runtime (CheerpJ) is downloaded from Leaning Technologies' CDN; your code is compiled and judged inside your browser and is never sent anywhere.

Found a mistake?

If something is wrong, unclear or out of date, please report it, and a source (LeetCode, the Java docs, a textbook) helps fix it fast. Open an issue on GitHub ↗

Built with

Next.js (static export), three.js with React Three Fiber and drei, Shiki for code highlighting, zod for content checks, CodeMirror for the editor, CheerpJ (Community Edition, free for personal use) with the OpenJDK 17 compiler (GPLv2 with Classpath Exception) for running Java, and JetBrains Mono NL (SIL Open Font License), the no-ligature build, so code always looks exactly as typed.