12 Core Algorithmic Patterns & 195 Practice Problems
- 1. Two Pointers (11 Paradigms, 35 Problems): Opposite ends, sorted pair sums, palindrome verification, trapping rain water, 3Sum, container with most water — includes the Sliding Window and Fast & Slow Pointers paradigms (contiguous subarray invariants, longest substrings, Floyd's cycle detection, linked list middle).
- 2. Binary Search (9 Paradigms, 14 Problems): Monotonic predicate partitioning (`feasible(x)`), boundary search, rotated sorted arrays, square roots, minimum ship capacity, median of two sorted arrays, matrix median on value range.
- 3. Bit Manipulation (5 Paradigms, 8 Problems): Counting set bits, XOR cancellation, bit reversal, bitmask pairing, maximum XOR with a bitwise trie.
- 4. Math & Geometry (8 Paradigms, 14 Problems): Sieve of Eratosthenes, integer/roman conversions, modular arithmetic, geometric simulation.
- 5. Tree/Graph Depth-First Search (10 Paradigms, 19 Problems): Path sums, lowest common ancestor, tree diameter, validating BSTs, tree DP (House Robber III, Binary Tree Cameras).
- 6. Tree/Graph Breadth-First Search (4 Paradigms, 11 Problems): Level-order traversals, shortest path in unweighted graphs, zig-zag traversals, rotting oranges, word ladders.
- 7. Graphs (10 Paradigms, 18 Problems): Topological sort, cycle detection, Dijkstra's algorithm, bipartite graph validation, network delay.
- 8. Backtracking (6 Paradigms, 14 Problems): Subsets, permutations, combinations, constraint satisfaction, pruning, N-Queens, Sudoku solver.
- 9. Dynamic Programming (11 Paradigms, 18 Problems): Memoization vs tabulation, 0/1 knapsack, unbounded knapsack, coin change, edit distance, longest common subsequence.
- 10. Heap / Priority Queue (9 Paradigms, 11 Problems): Running medians, top-k elements, interval scheduling, IPO, k-way merges.
- 11. Advanced Data Structures (6 Paradigms, 15 Problems): Trie (prefix tree), Union-Find (Disjoint Set Union), LRU Cache, LFU Cache, range-sum queries with updates.
- 12. Intervals & Stack / Miscellaneous (11 Paradigms, 18 Problems): Merge intervals, daily temperatures, largest rectangle in histogram, trapping rain water via stack.
4-Stage Deliberate Practice Framework
- Stage 1 (Compare & Learn): Read the pattern's mental model, loop invariant and Big-O proof, compare the pattern's template with the problem's solution side by side, and use the step-by-step debugger. The solution can also be shown in C#, Java, TypeScript, C++, Go or Rust for almost every problem.
- Stage 2 (Active Recall): Rebuild the solution from memory in a code editor, with an invariant checklist and optional progressive hints, and check it against a judged test suite.
- Stage 3 (AI Interview Coach, coming soon): A mock-interview stage still in development; for now, students go straight from Stage 2 to Stage 4.
- Stage 4 (Solve on Your Own): Solve a new problem on your own against a 15-minute timer; your code is graded against a test suite. Python and TypeScript run in your browser and C# runs on our server (design problems grade in Python and TypeScript).
Equipped with a spaced-repetition review hub on a fixed 7-step schedule (1 day up to 90 days), a two-pane Studio workspace, and study notes.
Pricing, Access & Commercial Terms
- Core Curriculum: 100% Free. No credit card required.
- Coins: New accounts get 40 free coins once, the daily claim adds +20 coins once a day, and inviting a friend earns you +25 coins when they complete their first practice step. These are the only ways to earn coins.
- AI Coach: Each AI coach question costs 1-5 coins, priced by the server according to its size.
- BYOK (Bring Your Own Key): AI coaching is free with your own Google Gemini, OpenAI or OpenRouter API key (BYOK).
- Extras: Coins also unlock extras for 24 hours: a Recall hint level (1 coin), a Solve solution reveal (2), a LeetCode problem card's solutions (1) and a pattern's extra canonical problems (1).
- Payments & Refunds: Nothing on the site is sold for money: coins can only be earned, never bought. There are no subscriptions or charges, so there is nothing to refund.
- Platform Operator: Hiđź‘‹WebEnterprise. Support & policies at hispeedalgo.com.