Train the pattern here.
Practice on LeetCode.

We are not another problem archive. We compress the gap between I read the problem and I see the trick from hours into minutes — with interactive animations that teach the mental model, not the syntax.

family · 3 sub-patterns

Two indices moving over the same sequence to maintain an invariant — converging, fast/slow, sliding window.

converging · fast-slow · sliding

Binary Search Variants

Boundary conditions are the field's largest unsolved teaching problem.

3 variants · O(log n)

Decision-tree expansion — uniquely suited to animation.

branching state machine

Monotonic Stack

Stack-state transitions are invisible in code, dramatic in animation. The stack is the protagonist.

next-greater · histogram

BFS / DFS on Gr DFS

A queue gives breadth, a stack gives depth — same algorithm, two traversal orders.

queue · stack · flood fill

family · 3 sub-patterns

Heap & Priority Queue

Cheap access to the extreme — min or max — while everything else stays loosely ordered. A family of three sub-patterns: top-k, two heaps, k-way merge.

top-k · two-heaps · k-way-merge

Tree Traversal

Every traversal visits every node once — only when relative to children's recursion differs.

preorder · inorder · postorder · level

Linked List Manipulation

Three pointers — prev, curr, next — let you rewrite a list in place without losing the tail.

reverse · merge · reorder

family · 5 sub-patterns

Dynamic Programming

A table fills itself, one cell at a time — each entry composed from a constant number of earlier ones. A family of five sub-patterns.

1-d · 2-d · knapsack · interval · state-machine

single · phase 3

Maintain a partition of N elements into disjoint components — merge two components in near-constant time, with the forest flattening itself on every query.

connected components · O(α(N))

single · phase 3

Store a set of strings on a tree of characters — words that share a prefix share a path, and a single 'end-of-word' flag separates stored words from in-flight prefixes.

prefix queries · autocomplete · word-search

single · phase 3

Topological Sort

Linearize a directed acyclic graph so every edge points left-to-right — each node enters the order only after every prerequisite has already left.

Kahn's algorithm · O(V + E) · cycle detection

family · 4 sub-patterns

Pay O(N) once to amortize every range query to O(1) — and flip it inside-out for cheap range updates. A family of four sub-patterns.

1-d · 2-d · difference · hash-map

family · 4 sub-patterns

Shortest Path

Find the shortest route from a source — the mechanism splits by what the edges carry. BFS for uniform costs, Dijkstra for non-negative, Bellman-Ford for negatives, Floyd-Warshall for all pairs.

BFS · Dijkstra · Bellman-Ford · Floyd-Warshall

Merge Intervals

Sort by start, then sweep — each interval either extends the current merge or commits it and starts a new one. The whole pattern lives in that one comparison.

sort + linear sweep · O(N log N)

Bit Manipulation

Treat integers as their bits, not their values — XOR a number against itself to zero it, against zero to keep it. The trick is recognizing when a problem has a hidden bit-level structure.

XOR · bit mask · O(N)

family · 2 sub-patterns

When values live in [0, N] or [1, N], the index is the pointer — each element already knows where it belongs. Swap to home, or sign-flip to mark seen, and a whole family of missing/duplicate problems collapses to O(N) and O(1) space.

index-as-pointer · O(N)