Every technical round. One place to prep for it.

Everything a software engineer needs to prep for technical rounds - coding, system design, and mock interviews, with AI-guided practice throughout.

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Interview guides and curated questions for top companies

GoogleMetaAmazonMicrosoftAppleNetflixOpenAIAnthropicStripeUberAirbnbDatabricksDatadogFigma+34 more

Coding Challenges

Coding questions from actual interviews at top companies

Python. More languages coming soon.

    Browse coding challenges
    A returnzero coding challenge: the problem brief beside an in-browser Python editor, with multi-level tests passing 3 out of 3.

    Android System Design

    Core concepts, framework walkthroughs, and detailed system design breakdowns for Android.

    Every breakdown includes an architecture diagram and trade-off tables. Run an AI mock interview on any problem to defend your design under pressure.

    • Core concepts and framework walkthroughs alongside every breakdown
    • Architecture diagrams and trade-off tables for each problem
    • AI mock interviews to defend your design
    Open a sample breakdown
    UI · screen + ViewModelDomain · use casesData · repositoryLocal Storage · durability + cacheNetwork · Background · SensorsBackend · wire surfaces onlyForecastScreenWeatherViewModel · StateFlow<UiState>ObserveForecastRefreshForecastResolveLocationWeatherRepositoryRoomforecast_cacheL1 InMemory(current forecast)RemoteDataSource(OkHttp + Retrofit)RefreshWorker(WorkManager)FusedLocationProviderClientWeather API · REST

    Guided Practice

    Design on a live canvas while an AI coach walks you through every step.

    Work an Android system design problem the way you would in the real round: sketch the architecture on the canvas, get coached one step at a time, and close with a debrief that scores your decisions.

    • Step-by-step prompts, from requirements to deep dives
    • Instant feedback on each part of your design
    • End-of-session debrief with a verdict and where to improve
    Try guided practice
    Guided practice for a mobile system design problem: an AI coach rail walks through each design step while you diagram the architecture on a live canvas.

    AI Interviewer

    Mock interviews on coding and system design problems, with feedback on how you communicate your approach.

    Get a Pass/Hold verdict with per-dimension scoring and specific feedback on what to improve.

    • Clarifying, coding, reviewing, and final verdict phases
    • Per-dimension scoring with specific feedback
    • Written summary after every session
    Start a mock interview
    An AI mock interview: your code and console on the left, a Pass verdict with per-dimension scoring and specific feedback on the right.
    Coming Next

    More technical rounds are being built.

    Distributed System Design

    Large-scale backend architecture - storage, caching, consistency, and the bottlenecks interviewers probe.

    AI Engineering

    RAG pipelines, model serving, and AI system design - the rounds that didn't exist five years ago.

    Infrastructure & Platform

    Platform engineering system design and core concepts - the layer below the application.

    Start free. Go deeper when you're ready.

    Core concepts and sample problems are free, no registration. Premium unlocks the full catalog, AI mock interviews, and every tool.

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