Learn DSA & System Design by Watching It Run

DSA

Watch the code and the data move together

Every problem opens with a concept page that builds the intuition, then a simulation that runs it. The highlighted line, the array, and the variable dashboard advance on the same step — so you see which line caused which change.

Arrays & StringsTwo PointersSliding WindowBinary SearchRecursionBinary TreesBFSDFSDynamic ProgrammingGraphsGreedySorting

Explore DSA →172 problems · 16 pattern groups

sliding-window · max sum (k=3)1 / 4 steps

2

0

1

5

2

1

3

4

2

5

Window [0…2] sum = 8 → new best!

</> CODE CONSOLEJAVA

for (int end = 0; end < n; end++) {
windowSum += nums[end];
if (end >= k - 1) {
best = max(best, windowSum);
windowSum -= nums[end - k + 1];
}
}

▣ STATE OBSERVER

WINDOW SUM8

BEST8

START0

LLD

Design patterns you can actually see working

Start from the naive version that hurts, then watch the refactor happen step by step. Class diagrams redraw as the design evolves, so the pattern arrives as a fix to a problem you already felt — not a definition to memorise.

OOP BasicsSOLID PrinciplesDesign PatternsState MachinesBooking SystemsBoard GamesCache DesignMoney & LedgerMarketplace

Explore LLD →57 lessons · 9 design families

observer · stock ticker3 SUBSCRIBED

</> CODE CONSOLEJS

class StockTicker {
observers = [];
subscribe(o) { this.observers.push(o); }
unsubscribe(o) { /* remove o */ }
setPrice(p) {
this.price = p;
for (const o of this.observers)
o.update(p);
}
}

▣ NOTIFICATIONS DELIVERED

Three screens are subscribed. The ticker holds a list, not names.

HLD

Scale a system from one server to a million users

A story, not a checklist. You start on a single server, feel it break, and add exactly one component per bottleneck — load balancer, cache, replicas, queues, shards — watching the topology animate into place at every stage.

Scaling JourneyFoundations of ScaleData LayerCommunicationReliabilityAdvanced TopicsReal System Designs

Explore HLD →24 chapters · monolith to web scale

system design · load balancingALL HEALTHY

▣ LIVE METRICS

HEALTHY NODES3 / 3

IN ROTATION3

DROPPED0

REQUEST LOG

Three healthy servers sit behind one address.

How it works

From concept to confident recall

01

Build the intuition

The concept page poses the problem, walks the corner cases, and explains why the approach works — before any code.

02

Watch it run

Step through execution with synced code highlighting, live variable state, and a plain-English description of every step. Narration optional.

03

Practice for real

Solve it yourself in the built-in multi-language code runner once the concept clicks.

Roadmaps

Told exactly what to do, every day

Pick a 1, 3, or 6 month plan and get a day-by-day schedule. Each week mixes two days of DSA, two of LLD, two of HLD and one revision day, with problems progressing easy → medium → hard.

Open the roadmaps →

1 Month3 Months6 Months

Week 1 · Arrays → OOP Basics → Monolith

Day 1DSAArrays: intuition + easy problems

Day 3LLDClasses, Objects & Encapsulation

Day 5HLDThe Monolith

Day 7REVISIONWeek 1 recap

Pricing

One payment. Yours forever.

Lifetime

$19once

All 253 lessons — DSA, LLD and HLD
Every concept page and simulation
Voice narration and the code runner
All roadmaps with progress tracking
Everything added later, at no extra cost

Get lifetime access

FAQ

Questions, answered

Is this a subscription?

No. One payment, lifetime access to everything — DSA, LLD, and HLD — including whatever gets added later.

Who is it for?

Anyone preparing for engineering interviews, and anyone who has read the theory but still can't picture what the algorithm or the system is actually doing.

Do I need to know a specific language?

No. Simulations render the same logical steps against JavaScript, Python, Java and C++ — pick whichever you think in.

How is this different from reading a tutorial?

A tutorial shows you the final code. Here you watch the code execute one line at a time with the data changing next to it, which is the part a static article can't show you.

Can I follow a structured plan?

Yes. The roadmaps lay out 1, 3, or 6 month schedules day by day, mixing DSA, LLD and HLD each week with a revision day.

Stop memorising algorithms.
Watch them run.