Learn Data Structures and Algorithms with Pictures (Free)

Data structures and algorithms (DSA) are the heart of most coding interviews, and the part many people dread. Tutorials tend to start with code. We wanted to start with something you already understand.

So we built free DSA practice on Nokku: short lessons that explain every idea with a real-world picture, diagrams you can step through, and original interview problems you solve right in your browser.

A stack is a pile of plates

Here's the idea in one picture. You put plates on top, and you take plates off the top. The last plate you added is the first one you take. That's a stack (last in, first out), and it's exactly how your browser's Back button and every function call in your program work.

Every topic starts like this:

• Arrays are a row of numbered lockers: if you know the number, you walk straight to it.
• Hash maps work like a coat check: the ticket tells you exactly which hook your coat is on.
• Queues are the line at a coffee shop: first come, first served.
• Linked lists are a train: each car only knows the car behind it.
• Trees are drawn upside down, with the root at the top. (Yes, really.)
• Graphs are a subway map: stations are nodes, tracks are edges.
• Binary search is how you find a name in a phone book: open the middle, throw away half.
• Dynamic programming is writing each answer down once so you never work it out twice.

Watch the algorithm move

Pictures explain the idea; diagrams show the algorithm. Most lessons and problems include diagrams you step through with Back and Next. Try this one: a sliding window looking for the shortest run of days whose steps add up to at least 7.

Shortest stretch with a total of at least 71 / 3

leftright

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Then solve real problems

Each of the 8 topics has 3 original practice problems, 24 in all, from easy to hard. You write your solution in Python or JavaScript in the page, run it against tests, and see exactly which cases pass. Your code runs privately in your own browser.

Stuck? You don't have to jump straight to the answer:

1. Brain Power questions make you think before you code.
2. The breakdown reveals hints one step at a time.
3. The solution comes with handwritten notes on the important lines, plus its time and space complexity.

A weekly leaderboard, and a free certificate

Sign in (free) to save your progress and earn points: 10 for an easy problem, 20 for a medium one and 30 for a hard one, plus points for quizzes. The leaderboard starts fresh every Monday, so anyone can reach the top.

Each week's top learners receive a free, verifiable certificate they can download and add to their LinkedIn profile. To keep it fair, problems solved after viewing the solution, or by pasting it, don't earn points.

Start learning

It's free, and you don't need an account to start reading or solving.

• Start with Topic 1: Arrays and hashing
• See all 8 topics
• Check this week's leaderboard

Preparing for a specific company? Our interview prep guides explain how Google, Amazon, Microsoft, Meta, Adobe and Atlassian run their interviews, from their own published information.