System design & algorithms, taught through interactive animations.
Every concept comes with a step-by-step visual: watch requests flow, caches hit and miss, and arrays sort — with the code highlighted line by line beside it. Step through at your own pace, then walk into your interview knowing exactly how it works.
System Design
18 lessonsScale a system from one server to millions of users — and design real products.
Client–Server Basics
How a request travels: DNS, a server, and a database.
Open lesson Chapter 2Vertical vs Horizontal Scaling
Scale up (bigger box) vs scale out (more boxes).
Open lesson Chapter 3Load Balancing Algorithms
Round-robin, least-connections, and consistent hashing.
Open lesson Chapter 4Caching Strategies
Cache hits & misses; cache-aside, write-through, write-back.
Open lesson Chapter 5Database Sharding & Replication
Copy data for reads; split data to scale writes.
Open lesson Chapter 6System Communication
REST, gRPC, WebSockets, webhooks, and message queues.
Open lessonData Structures & Algorithms
71 lessonsSee how each structure and algorithm works, step by step, with the code beside it.
Arrays & Big-O
Contiguous memory and the cost model: O(1) access vs O(n) scan.
Open lesson Lesson 2Two Pointers
Walk two pointers inward to find a pair in one pass.
Open lesson Lesson 3Sliding Window
Slide a fixed window in one pass instead of rechecking.
Open lesson Lesson 4Binary Search
Halve a sorted range each step — O(log n).
Open lesson Lesson 5Linked Lists
Traverse a chain of nodes, then reverse it in place.
Open lesson Lesson 6Stacks & Queues
LIFO vs FIFO: pop the last one in, or remove the first.
Open lessonCoding Problems
484 lessonsSolve classic interview problems — the approach, the animation, and the code, line by line.
Two Sum
Find two numbers that add up to a target — in one pass with a hash map.
Open lesson Problem 2Valid Parentheses
Match brackets with a stack — every closer must pop its opener.
Open lesson Problem 3Reverse Linked List
Flip each next-pointer backwards with three pointers, in one pass.
Open lesson Problem 4Best Time to Buy and Sell Stock
One pass: track the lowest price so far and the best profit so far.
Open lesson Problem 5Maximum Subarray
Kadane's scan: extend or restart the running sum to find the largest slice.
Open lesson Problem 6Maximum Depth of Binary Tree
Recurse: a node's depth is 1 + the deeper of its two children.
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