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Sorting and the foundation of algorithm state

See how loops walk array indices, how swaps move values, and how the same algorithm reads differently in JavaScript and Python.

Templates: Bubble sort · Bubble sort · Bubble sort

Binary search and narrowing the search space

Left/right and slow/fast pointers shrink what an algorithm has to look at. Learn binary search plus pointer techniques on lists.

Templates: Binary search · Binary search · Middle of a linked list · Nth node from the end

Subarray and substring problems in one pass

A window that grows and shrinks across the input solves many O(n) substring problems. Watch the window bounds and the map of counts move together.

Templates: Sliding window · Longest repeating character replacement

Decompose a problem into smaller versions of itself

Watch recursive calls push onto the call stack, branch through the graph, and unwind. The checkpoint quizzes here test whether you can predict each call's effect.

Templates: Recursive depth-first search

Recursive structure, level by level

Binary trees are the classic recursion workout: mirror the tree, compute depth, and watch node pointers move down each branch.

Templates: Invert binary tree · Maximum depth of binary tree

Traversal order and visited sets

DFS and BFS order decide everything on graphs. Watch the frontier grow, nodes get visited, and grid problems behave like graph problems.

Templates: Depth-first search · Number of islands

Memoize the past to answer the future

Coming soon — DP tables and memoization trees are in the works. Check back for the first templates.

Rewiring pointers instead of moving values

Reversing, merging, and cycle detection all come down to carefully reassigning next pointers. Watch the node links rewire step by step.

Templates: Reverse a linked list · Detect cycle (Floyd's algorithm) · Merge two sorted lists

Ordering with a stack, lookup with a map

A stack keeps the right order for balanced parentheses; a hash map turns a two-pass search into a single pass. See both structures in action.

Templates: Valid parentheses · Two sum · Two sum

Row, column, and cell coordinates

Two-dimensional state: spiral traversals, marking rows and columns, and indexing cells by [row][col].

Templates: Spiral matrix · Set matrix zeroes