LOW-LEVEL DESIGN PRACTICE

Practice LLD machine coding and concurrency problems.

One problem. One source file. Real feedback.

JAVA · PYTHON · C++

SplitwiseService.java

public final class SplitwiseService {
  public List<Settlement> settle(...) {
    return policy.reduce(balances);
  }
}

4 CUSTOM CASES · READY TO RUN

How it works

Choose a problem

Filter by concepts, difficulty, or company.

Write one solution

Work in a focused, single-file workspace.

Run and review

Use manual cases and inspect past submissions.

PRACTICE PATHS

Choose a focused sequence.

Follow an ordered path or begin with a complete free problem.

LLD MACHINE CODINGBuild complete low-level designs.

Move from state and invariants to policies, recovery, and full implementations.

Open practice path →CONCURRENCY INTERVIEWSReason about overlapping operations.

Practice shared state, coordination, scheduling, and transactional behavior.

Open practice path →

WHAT YOU PRACTICE

Model behavior, change, and concurrency.

Each problem gives you a behavioral contract to model, implement, and exercise under explicit constraints.

Model behavior

Turn requirements into cohesive objects, contracts, and invariants.

Design for change

Use policies and extension points without building unnecessary abstractions.

Handle concurrency

Exercise shared state with explicit threads, operations, and typed arguments.

Reason under limits

Work with real runtime, memory, process, and output constraints.

THE WORKSPACE

Code, testcases, and results in one workspace.

Keep the problem contract, implementation, testcase builder, and judge feedback in one focused surface.

Language-aware templates for Java, Python, and C++
Typed public and custom testcase forms generated from problem metadata
Run one custom case or the complete public and custom case set
Inspect structured results, stdout, stderr, runtime, and memory
Autosave code and testcase drafts independently for every runtime

RUN · SUBMIT · REVIEW

See what ran and preserve what you submitted.

RUN

Run visible cases

Run public cases and the custom cases you create, with inputs, stdout, stderr, runtime, and memory visible.

SUBMIT

Check hidden cases

Submit the same source against public and hidden cases in an isolated judge.

REVIEW

Review fixed snapshots

Reopen the immutable source snapshot for each submission and compare outcomes over time.

PUBLIC PROBLEM CATALOG

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