See exactly how returnzero_ works.
See how returnzero works - learn the concepts, practice in an in-browser sandbox, run a system design mock interview, and lock it in with spaced-repetition review.
One loop, four moves: learn the concept, practice it in a real sandbox, defend it in a live mock interview, and keep it with spaced repetition. Every tool runs right in your browser.
Run every tool yourself
Run code against a test suite, flip a review card, drive a mock interview, and see your plan on the dashboard.
1def credit_tracker(events):2 # Track allocations and client accounts3 credits = {}4 for ev in events:5 if ev["type"] == "allocate":6 credits[ev["client"]] = credits.get(ev["client"], 0) + ev["amount"]7 elif ev["type"] == "consume":8 current = credits.get(ev["client"], 0)9 if current >= ev["amount"]:10 credits[ev["client"]] = current - ev["amount"]11 return creditsRun the tests to verify the implementation.
Learn, practice, interview, review
Every track runs on the same four steps - learn the concept, build it in code, defend it in a mock interview, and keep it with spaced repetition.
Learn the concepts
Start from deep breakdowns - mobile and backend system design, AI engineering, infrastructure, and DSA - with the real trade-offs and capacity math worked end to end.
Practice in the sandbox
Solve build-style challenges in an in-browser editor. Write code, run the real test suite, and iterate - Python, JavaScript, and TypeScript, no setup.
Run a mock interview
A live AI interviewer runs a full round: clarify the problem, code your solution, then defend complexity and edge cases - and get a written performance summary.
Lock it in with review
Active-recall cards scheduled by the SM-2 algorithm resurface what you're about to forget, so the concepts stick past interview day.
See the interviewer's own scorecard
Every coding challenge ships an interviewer guide - the signals they look for, the green / yellow / red flags, and the 1-5 rubric. Stop guessing what good looks like.
Signals the interviewer looks for
- Names the delimiter problem unprompted before writing any code (
any character can appear in the strings, so I can't use a fixed delimiter) - Describes
length-prefix framingin one sentence and starts coding immediately - Recognizes that Level 2's adversarial inputs need no code change - the length-prefix scheme already handles them
- Proposes a delimiter scheme first but pivots quickly to length-prefix after one question
- Implements length-prefix correctly but adds unnecessary escaping for the colon character
- Uses
str.split(":", 1)in the decoder - works for simple cases but fails when the payload starts with a string like"5:hello"
- Proposes a delimiter scheme and adds escaping instead of switching to length-prefix after the delimiter problem is pointed out
- Uses
len(s.encode("utf-8"))as the length prefix - counts bytes, not characters; breaks for multi-byte unicode - Implements
decodewithsplitand assumes the colon separator is unique in the payload
Scored 1 - 5 on five dimensions
Named the delimiter problem and length-prefix solution before writing any code. No prompting required.
Still proposing delimiter + escaping after all hints; never arrived at a clean length-prefix decoder.
Excerpt from the Encode and Decode Strings interviewer guide.
Open an actual problem
Real pages from the catalog, open to read right now - here are three to start with.
Weather App
A forecast app that loads instantly from cache, shows an honest offline state, and refreshes in the background.
Encode and Decode Strings
Serialize a list of arbitrary strings into a single string and parse it back. Any character is allowed in the strings, including the delimiters - the format must round-trip them all.
+3The Android App Template
The architecture every Android system design inherits. A reference to most app designs.
Ready to go deep?
Two free problems of every type, no registration. Premium unlocks the full catalog and every tool.