Free course · EASY → HARD

Algorithms

200 reusable patterns, each taught twice and then proved in a Rust challenge

20 units · 600 lessons · every lesson is open

Linear Scans & State

One pass, explicit state, and the invariants behind later patterns

Hash Maps & Sets

Trade memory for direct membership, grouping, and complement lookup

Prefix Sums & Range Queries

Precompute cumulative information so repeated range work becomes cheap

Two Pointers

Use ordering or a maintained boundary to avoid nested scans

Sliding Window

Maintain exactly the state of one contiguous region

Fixed-Size Window: Mental Model12 min Fixed-Size Window: When to Use It12 min Challenge: Fixed-Size Window15 min Variable-Size Window: Mental Model12 min Variable-Size Window: When to Use It12 min Challenge: Variable-Size Window15 min Last-Seen Unique Window: Mental Model12 min Last-Seen Unique Window: When to Use It12 min Challenge: Last-Seen Unique Window15 min At-Most-K Window: Mental Model12 min At-Most-K Window: When to Use It12 min Challenge: At-Most-K Window15 min Exactly-K by Difference: Mental Model12 min Exactly-K by Difference: When to Use It12 min Challenge: Exactly-K by Difference15 min Frequency-Balanced Window: Mental Model12 min Frequency-Balanced Window: When to Use It12 min Challenge: Frequency-Balanced Window15 min Minimum Cover Window: Mental Model15 min Minimum Cover Window: When to Use It15 min Challenge: Minimum Cover Window15 min Monotonic Deque Window: Mental Model15 min Monotonic Deque Window: When to Use It15 min Challenge: Monotonic Deque Window15 min Rolling Mean and Variance: Mental Model15 min Rolling Mean and Variance: When to Use It15 min Challenge: Rolling Mean and Variance15 min Sliding Median with Two Heaps: Mental Model15 min Sliding Median with Two Heaps: When to Use It15 min Challenge: Sliding Median with Two Heaps15 min

Stack, Queue & Monotonic Patterns

Choose the removal order that matches the dependency

Delimiter Stack: Mental Model12 min Delimiter Stack: When to Use It12 min Challenge: Delimiter Stack15 min Postfix Evaluation: Mental Model12 min Postfix Evaluation: When to Use It12 min Challenge: Postfix Evaluation15 min Min Stack: Mental Model12 min Min Stack: When to Use It12 min Challenge: Min Stack15 min Queue from Two Stacks: Mental Model12 min Queue from Two Stacks: When to Use It12 min Challenge: Queue from Two Stacks15 min Next Greater Element: Mental Model12 min Next Greater Element: When to Use It12 min Challenge: Next Greater Element15 min Monotonic Increasing Stack: Mental Model12 min Monotonic Increasing Stack: When to Use It12 min Challenge: Monotonic Increasing Stack15 min Monotonic Decreasing Stack: Mental Model12 min Monotonic Decreasing Stack: When to Use It12 min Challenge: Monotonic Decreasing Stack15 min Circular Monotonic Stack: Mental Model15 min Circular Monotonic Stack: When to Use It15 min Challenge: Circular Monotonic Stack15 min Shunting-Yard Parsing: Mental Model15 min Shunting-Yard Parsing: When to Use It15 min Challenge: Shunting-Yard Parsing15 min Largest Histogram Rectangle: Mental Model15 min Largest Histogram Rectangle: When to Use It15 min Challenge: Largest Histogram Rectangle15 min

Sorting & Selection

Order everything, or isolate only the rank the problem needs

Binary Search

Exploit a monotone decision boundary, not only a sorted Vec

Exact Binary Search: Mental Model12 min Exact Binary Search: When to Use It12 min Challenge: Exact Binary Search15 min Lower Bound: Mental Model12 min Lower Bound: When to Use It12 min Challenge: Lower Bound15 min Upper Bound: Mental Model12 min Upper Bound: When to Use It12 min Challenge: Upper Bound15 min First and Last Position: Mental Model12 min First and Last Position: When to Use It12 min Challenge: First and Last Position15 min Rotated Sorted Search: Mental Model12 min Rotated Sorted Search: When to Use It12 min Challenge: Rotated Sorted Search15 min Peak Binary Search: Mental Model12 min Peak Binary Search: When to Use It12 min Challenge: Peak Binary Search15 min Flattened Matrix Search: Mental Model12 min Flattened Matrix Search: When to Use It12 min Challenge: Flattened Matrix Search15 min Binary Search on the Answer: Mental Model15 min Binary Search on the Answer: When to Use It15 min Challenge: Binary Search on the Answer15 min Exponential Search: Mental Model15 min Exponential Search: When to Use It15 min Challenge: Exponential Search15 min Partition Median: Mental Model15 min Partition Median: When to Use It15 min Challenge: Partition Median15 min

Linked List Techniques

Rewire ownership-like links without losing the unread suffix

Tree Traversal & BST

Choose traversal order from when a node's result becomes known

Heaps & Streaming

Keep only the next best candidate instead of fully ordering the world

Binary Heap: Mental Model12 min Binary Heap: When to Use It12 min Challenge: Binary Heap15 min Top-K with a Min-Heap: Mental Model12 min Top-K with a Min-Heap: When to Use It12 min Challenge: Top-K with a Min-Heap15 min Kth Statistic in a Stream: Mental Model12 min Kth Statistic in a Stream: When to Use It12 min Challenge: Kth Statistic in a Stream15 min K-Way Heap Merge: Mental Model12 min K-Way Heap Merge: When to Use It12 min Challenge: K-Way Heap Merge15 min Bounded Heap for K Closest: Mental Model12 min Bounded Heap for K Closest: When to Use It12 min Challenge: Bounded Heap for K Closest15 min Median from Two Heaps: Mental Model15 min Median from Two Heaps: When to Use It15 min Challenge: Median from Two Heaps15 min Lazy Heap Deletion: Mental Model15 min Lazy Heap Deletion: When to Use It15 min Challenge: Lazy Heap Deletion15 min Priority Scheduling: Mental Model15 min Priority Scheduling: When to Use It15 min Challenge: Priority Scheduling15 min Meeting Rooms Heap: Mental Model15 min Meeting Rooms Heap: When to Use It15 min Challenge: Meeting Rooms Heap15 min Huffman Merge Heap: Mental Model15 min Huffman Merge Heap: When to Use It15 min Challenge: Huffman Merge Heap15 min

DFS, BFS & Graph Traversal

Represent edges explicitly, then control frontier and visited state

Shortest Paths, MST & Flow

Match edge guarantees to the correct shortest-path or connectivity proof

Dijkstra: Mental Model12 min Dijkstra: When to Use It12 min Challenge: Dijkstra15 min 0–1 BFS: Mental Model12 min 0–1 BFS: When to Use It12 min Challenge: 0–1 BFS15 min DAG Shortest Path: Mental Model12 min DAG Shortest Path: When to Use It12 min Challenge: DAG Shortest Path15 min Prim Minimum Spanning Tree: Mental Model12 min Prim Minimum Spanning Tree: When to Use It12 min Challenge: Prim Minimum Spanning Tree15 min Kruskal Minimum Spanning Tree: Mental Model12 min Kruskal Minimum Spanning Tree: When to Use It12 min Challenge: Kruskal Minimum Spanning Tree15 min Bellman–Ford: Mental Model15 min Bellman–Ford: When to Use It15 min Challenge: Bellman–Ford15 min Floyd–Warshall: Mental Model15 min Floyd–Warshall: When to Use It15 min Challenge: Floyd–Warshall15 min A* Search: Mental Model15 min A* Search: When to Use It15 min Challenge: A* Search15 min Bidirectional BFS: Mental Model15 min Bidirectional BFS: When to Use It15 min Challenge: Bidirectional BFS15 min Dinic Maximum Flow: Mental Model15 min Dinic Maximum Flow: When to Use It15 min Challenge: Dinic Maximum Flow15 min

Backtracking & Exhaustive Search

Build a candidate, reject early, and undo exactly what you changed

Subset Generation: Mental Model12 min Subset Generation: When to Use It12 min Challenge: Subset Generation15 min Permutation Backtracking: Mental Model12 min Permutation Backtracking: When to Use It12 min Challenge: Permutation Backtracking15 min Combination Backtracking: Mental Model12 min Combination Backtracking: When to Use It12 min Challenge: Combination Backtracking15 min Combination Sum: Mental Model12 min Combination Sum: When to Use It12 min Challenge: Combination Sum15 min Valid Parentheses Generation: Mental Model12 min Valid Parentheses Generation: When to Use It12 min Challenge: Valid Parentheses Generation15 min Grid Word Search: Mental Model12 min Grid Word Search: When to Use It12 min Challenge: Grid Word Search15 min Palindrome Partitioning: Mental Model12 min Palindrome Partitioning: When to Use It12 min Challenge: Palindrome Partitioning15 min N-Queens: Mental Model15 min N-Queens: When to Use It15 min Challenge: N-Queens15 min Sudoku Constraint Search: Mental Model15 min Sudoku Constraint Search: When to Use It15 min Challenge: Sudoku Constraint Search15 min Meet in the Middle: Mental Model15 min Meet in the Middle: When to Use It15 min Challenge: Meet in the Middle15 min

Greedy & Interval Methods

Make a locally safe commitment and prove what future choices remain

Activity Selection: Mental Model12 min Activity Selection: When to Use It12 min Challenge: Activity Selection15 min Merge Intervals: Mental Model12 min Merge Intervals: When to Use It12 min Challenge: Merge Intervals15 min Insert into Sorted Intervals: Mental Model12 min Insert into Sorted Intervals: When to Use It12 min Challenge: Insert into Sorted Intervals15 min Sweep-Line Events: Mental Model12 min Sweep-Line Events: When to Use It12 min Challenge: Sweep-Line Events15 min Greedy Reach Frontier: Mental Model12 min Greedy Reach Frontier: When to Use It12 min Challenge: Greedy Reach Frontier15 min Greedy Layered Jumps: Mental Model12 min Greedy Layered Jumps: When to Use It12 min Challenge: Greedy Layered Jumps15 min Gas Station Reset: Mental Model12 min Gas Station Reset: When to Use It12 min Challenge: Gas Station Reset15 min Last-Occurrence Partition: Mental Model12 min Last-Occurrence Partition: When to Use It12 min Challenge: Last-Occurrence Partition15 min Deadline Profit Scheduling: Mental Model15 min Deadline Profit Scheduling: When to Use It15 min Challenge: Deadline Profit Scheduling15 min Interval Stabbing: Mental Model15 min Interval Stabbing: When to Use It15 min Challenge: Interval Stabbing15 min

1D Dynamic Programming

Define one state, its transition, base cases, and evaluation order

2D & Advanced Dynamic Programming

Model two dimensions, intervals, subsets, and compressed state spaces

Grid Path DP: Mental Model12 min Grid Path DP: When to Use It12 min Challenge: Grid Path DP15 min Subset Sum DP: Mental Model12 min Subset Sum DP: When to Use It12 min Challenge: Subset Sum DP15 min Longest Common Subsequence: Mental Model12 min Longest Common Subsequence: When to Use It12 min Challenge: Longest Common Subsequence15 min Edit Distance: Mental Model12 min Edit Distance: When to Use It12 min Challenge: Edit Distance15 min Longest Palindromic Subsequence: Mental Model12 min Longest Palindromic Subsequence: When to Use It12 min Challenge: Longest Palindromic Subsequence15 min Matrix Chain DP: Mental Model15 min Matrix Chain DP: When to Use It15 min Challenge: Matrix Chain DP15 min Interval DP: Mental Model15 min Interval DP: When to Use It15 min Challenge: Interval DP15 min Digit DP: Mental Model15 min Digit DP: When to Use It15 min Challenge: Digit DP15 min Bitmask DP: Mental Model15 min Bitmask DP: When to Use It15 min Challenge: Bitmask DP15 min Tree DP: Mental Model15 min Tree DP: When to Use It15 min Challenge: Tree DP15 min

Tries & String Matching

Respect bytes versus characters, then exploit prefixes and borders

Character Frequency: Mental Model12 min Character Frequency: When to Use It12 min Challenge: Character Frequency15 min String Two Pointers: Mental Model12 min String Two Pointers: When to Use It12 min Challenge: String Two Pointers15 min Run-Length Encoding: Mental Model12 min Run-Length Encoding: When to Use It12 min Challenge: Run-Length Encoding15 min Knuth–Morris–Pratt: Mental Model12 min Knuth–Morris–Pratt: When to Use It12 min Challenge: Knuth–Morris–Pratt15 min Z Algorithm: Mental Model12 min Z Algorithm: When to Use It12 min Challenge: Z Algorithm15 min Rabin–Karp: Mental Model12 min Rabin–Karp: When to Use It12 min Challenge: Rabin–Karp15 min Prefix Trie: Mental Model12 min Prefix Trie: When to Use It12 min Challenge: Prefix Trie15 min Aho–Corasick: Mental Model15 min Aho–Corasick: When to Use It15 min Challenge: Aho–Corasick15 min Suffix Array: Mental Model15 min Suffix Array: When to Use It15 min Challenge: Suffix Array15 min Manacher Palindrome Radii: Mental Model15 min Manacher Palindrome Radii: When to Use It15 min Challenge: Manacher Palindrome Radii15 min

Math, Bits & Combinatorics

Replace simulation with algebraic structure and binary identities

Advanced Data Structures

Compose range, ordering, and decomposition techniques for hard constraints