Algorithms you can see, then play.
Most people find data structures hard because they're invisible. Here, every algorithm moves — step by step, in plain English — and the moment it clicks, you get a problem to prove it.
14 concepts · 6 problems · no sign-up, ever
Your learning trail
A path built for newbies — each stop unlocks the next. See the full map →
Watch it move
Every step animates with a plain-English line telling you exactly what just happened — and why.
Play with it
Pause, rewind, slow it down, or feed in your own numbers and re-run. It's a sandbox, not a slideshow.
Then prove it
See brute → best for real problems, then write your own solution in the in-browser editor.
All concepts
Linear Search
+40Check each element one by one until you find the target. The O(n) baseline.
Binary Search
+50Find a target in a sorted array by repeatedly halving the search space.
Two Pointers
+60Find a pair summing to a target in a sorted array by converging two pointers.
Hashing (Hash Set)
+70Store values in buckets by value % size for O(1) average lookup.
Stack & Queue
+60LIFO vs FIFO: a stack pops the newest, a queue dequeues the oldest.
Linked List Traversal
+60Follow next pointers from head to null — a pointer is a reference to a node.
Bubble Sort
+50Sort by repeatedly swapping adjacent out-of-order pairs until settled.
Insertion Sort
+50Grow a sorted prefix by inserting each next element into its correct spot.
Merge Sort
+90Divide the array in half, sort each half, then merge them. O(n log n).
Recursion: Factorial
+60See recursion as a call stack: calls push on the way down, pop on the way up.
Tree Traversal (In-order)
+80In-order traversal of a BST (left → node → right) visits values in sorted order.
DP: Climbing Stairs
+70Count ways to climb n stairs (1 or 2 at a time) by filling a DP table.
Breadth-First Search
+80Explore a graph level by level using a queue — nearest nodes first.
Depth-First Search
+80Explore a graph by going deep down one path before backtracking.