Build real systems. Step by step. In your browser.

Not slides, not videos. You write the code, run it against real tests, watch your own solution physically execute in spatial visualizers, and build complete production architectures.

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Stepwise Data Structure Visualizer

Watch data structures move in physical space

Not a pre-recorded animation. Elements actually lift, translate along arcs, and land in their new slots. Step forward and backward line-by-line with synchronized algorithmic code across 5 languages.

Launch Visualizer

Two-Pointer In-Place ReversalArray Operation

Step 1 of 9

reverse_array.ts

1function reverseArray(arr: number[]): void {

▶ let left = 0, right = arr.length - 1;

3 while (left < right) {

4 [arr[left], arr[right]] = [arr[right], arr[left]];

5 left++; right--;

6 }

Execution Trace: Initialize left pointer at index 0 and right pointer at index 4.

Array Memory Buffer · 5 contiguous slotsZero Latency WASM

▲ left

▲ right

Time: O(N)Auxiliary Space: O(1) In-Place

Pointer Convergence: left < right

Visualize 8 core data structures (Arrays, Linked Lists, Stacks, Queues, Binary Trees, BSTs, Heaps, and Hash Maps) with full step-by-step controls.Open Full Visualizer

Interactive System Design

Architect real-world scale before you get interviewed

Draw system topologies, run automated reliability validation, and estimate storage, QPS, and network throughput with back-of-the-envelope calculators.

Scored Labs Open Canvas

Architecture Whiteboard: Scalable Distributed Web5 Nodes · 4 Smart Connectors

Go Core MicroservicesOperational

Stateless Auto-scaling Cluster (3 AZs)

Throughput: 48.0k/s

P99 Latency: 32ms

CPU Load: 68%

Design your own topologies, calculate QPS/bandwidth math, and pass scored architecture labs.

Scored Labs Open Architecture Studio

Three tiers, one loop

Most sites give you one of these. The gap between reading about an LRU cache and being able to write one is where people get stuck, so the path runs all the way through — and nothing is locked, so you can start wherever you like.

1. Read itLessons
20 DSA topics and 11 on system design. Each one explains the pattern, then animates a real run of it — your code, stepping line by line.
Start with arrays →
2. Practise itProblems
42 single-function problems across 18 topics, easiest first. Progressive hints, a real test runner, and hidden cases so a solution fitted to the examples does not pass.
Try Two Sum →
3. Build itChallenges
8 multi-step builds, 32 steps in all. An LRU cache, a rate limiter, undo and redo — assembled across several files, one rule at a time.
Build an LRU cache →

The part that is actually different

Everything runs in your browser — Python, JavaScript, TypeScript, Go, C++, and Java on real interpreters compiled to WebAssembly. Nothing is queued on a server, so nothing costs anything to run.

Watch your own code run
Not a recorded animation of someone else's solution. The visualiser traces the code you wrote — array reads and writes, pointers, the line currently executing, highlighted in your editor as it steps.
Design labs that score reasoning
6 scenarios worked the way an interview goes — requirements, estimation, API, data model, scaling, bottleneck — with a scorecard naming which of the six went worst.

Mock interviews that feel real

Timed rounds with a Socratic AI interviewer that listens, pushes back, and scores you — not a quiz, an actual conversation about your approach.

DSA Rounds
45-minute timed DSA interviews with live code editing, progressive hints, and a detailed scorecard across communication, correctness, and complexity analysis.
System Design Rounds
Full system design sessions: requirements, API design, data modelling, component architecture, and bottleneck analysis — scored across the 10-dimensional architectural reference.
Post-Round Debrief
After each round, a detailed scorecard shows what went well and what to practise. Decision points are highlighted, and each section maps back to the lesson that covers it.

Try a mock interview

8 company interview guides

Verified, first-party engineering interview guides for Amazon, Google, Meta, Microsoft, Apple, Netflix, and Uber — sourced from official engineering blogs, recruiter documentation, and public tech talks.

Amazon

Google

Learn from the community

After you solve a problem, see how others approached it. Share your own solution, discuss edge cases, and compare complexity trade-offs — all within the workspace.

Peer Solutions
A completion-gated gallery of community approaches — with Big-O tags, language filters, upvoting, and spoiler protection. See the hash map approach next to the two-pointer one.
Problem Discussions
Threaded Q&A on every problem. Edge-case callouts, optimization follow-ups, and intuition discussions — tagged and filterable so the signal stays high.
Leaderboard
Ranked by problems solved, streak length, challenges built, and total submissions. See where you stand and what the pace looks like at the top.
View leaderboard →

What is in it

31lessons 42practice problems 32build steps 89reference terms 6design labs 8company guides

Ready to start cracking?

Jump in and start exploring — when you're ready to track your streaks, save progress, and climb the leaderboard, create a free account in seconds.

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