Two Pointers
Opposite-direction, fast-slow, sliding compare.
610problems
4 varsPattern-Based Learning
Master DSA by Pattern
Learn the skeleton once, apply to hundreds of problems. Each pattern breaks down into variations - the template stays 90% the same, only the details change.
Part I - Arrays & Pointers
Sliding Window
Turn O(n²) subarray loops into O(n) with a moving window.
610problems
5 varsBinary Search
Narrow the search space in half each step - the O(log n) hammer.
3,709problems
5 varsPrefix Sum
Precompute running sums for O(1) range queries and subarray counting.
4,313problems
Learn →Two Sum Family
HashMap-based pair/triple sum lookups. Two Sum, Three Sum, Four Sum, best time to buy/sell.
4,313problems
Learn →Array Tricks
Cyclic rotation, reversal-based operations, in-place swaps, and the Dutch flag.
4,313problems
Learn →Difference Array
O(1) range updates with O(n) reconstruction. Range increment/decrement queries.
4,313problems
Learn →Contribution Technique
Count how many subarrays each element participates in.
4,313problems
Learn →Binary Search on Answer
When the answer space is monotonic, binary search the answer and check feasibility.
4,313problems
Learn →Permutation Patterns
Next permutation, permutation ranking, generating permutations systematically.
4,313problems
Learn →Word Break Variants
Segment strings using dictionary words, with DP and memoization.
4,313problems
Learn →Shortest Common Supersequence
Shortest string that has two given strings as subsequences.
4,313problems
Learn →Kadane's Algorithm Variants
Maximum subarray sum in O(n), with circular, 2D, and product variants.
4,313problems
Learn →Two Pointers - Advanced
Multiple pointers, three-way partition, k-sum beyond three.
4,313problems
Learn →Cyclic Sort
Place each number at its correct index in O(n). For problems with numbers 1..n.
4,313problems
Learn →Part II - Linked Structures
Linked List
Reverse, merge, detect cycles, find middle, LRU cache.
1,135problems
Learn →Trees
Binary trees, BSTs, traversals (in/pre/post/level), LCA, diameter.
2,714problems
Learn →Advanced Trees
Fenwick tree, segment tree, sqrt decomposition, Mo's algorithm, treap.
924problems
Learn →Sparse Table
O(1) range min/max/gcd queries with O(n log n) preprocessing.
924problems
Learn →Segment Tree with Lazy Propagation
Range updates and range queries in O(log n). Lazy tags for assignment/additive updates.
924problems
Learn →Cycle Detection
Floyd's tortoise and hare, Brent's algorithm. Cycle in LL, arrays, functional graphs.
1,135problems
Learn →Tree Construction
Build trees from traversals, serialization, construct BST from preorder.
2,714problems
Learn →Lowest Common Ancestor
Binary lifting, Euler tour + RMQ, tarjan's offline LCA.
1,290problems
Learn →DSU on Tree (Sack)
Small-to-large merging on trees for offline subtree queries in O(n log n).
1,290problems
Learn →Tree Diameter
Two BFS/DFS technique. Find longest path in any tree. Center(s) of tree.
1,290problems
Learn →Offline LCA
Tarjan's offline algorithm for batch LCA queries using DSU.
1,290problems
Learn →Tree Isomorphism
AHU algorithm, rooted/unrooted tree isomorphism, canonical form.
1,290problems
Learn →Persistent Union-Find
Union-find with rollback capability for offline queries.
1,290problems
Learn →Binary Lifting
Jump pointers for ancestors, LCA, path queries in O(log n).
2,714problems
Learn →Segment Tree Basics
Point updates, range queries, building and querying segment trees.
2,714problems
Learn →Tree Path Problems
Path sum queries, max/min edge on path, path aggregates with binary lifting.
2,714problems
Learn →Iterative Tree Traversal
Morris traversal, stack-based pre/in/post-order without recursion.
2,714problems
Learn →Part III - Hashing & Auxiliary Structures
Heap / Priority Queue
Min/max heap, top-k, median finding, merge k-sorted, Huffman coding.
496problems
Learn →Trie
Prefix tree for dictionary, autocomplete, spell check, IP routing, XOR max pair.
158problems
Learn →Monotonic Stack
Next greater/smaller element, largest rectangle in histogram, trapping rain water.
3,426problems
Learn →Hashmap Patterns
Frequency counting, anagram grouping, subarray sum equals k, longest consecutive sequence.
4,313problems
Learn →Stack & Queue
Min stack, queue by stacks, circular deque, sliding window max (deque).
1,290problems
Learn →Counting Patterns
Counting sort, bucket sort, frequency array, cumulative distribution, pigeonhole.
4,313problems
Learn →K-th Element
Quickselect, median of medians, k-th smallest in sorted matrix, k-th largest in stream.
496problems
Learn →Monotonic Queue
Deque maintaining monotonic order for sliding window min/max, range min queries.
4,313problems
Learn →Two Heaps
Median from data stream, sliding window median, IPO capital, schedule tasks.
496problems
Learn →Bracket Sequences
Valid parentheses generation, longest valid parentheses, minimum add to make valid.
4,313problems
Learn →Amortized Patterns
Why O(n) operations can cost O(1) each on average (two-pointer, stack, deque).
4,313problems
Learn →Expression Parsing
Shunting yard, recursive descent, prefix/postfix conversion, basic calculator.
4,313problems
Learn →Trie XOR
Max XOR pair/subarray using binary trie. Properties of XOR on bits.
4,313problems
Learn →K-Way Merge
Merge k-sorted arrays/lists, smallest range covering k lists, merge k-sorted iterators.
4,313problems
Learn →Part IV - Core Algorithms
Dynamic Programming
Memoization, tabulation, state definition, transitions. The universal hammer for optimal substructure.
4,313problems
6 varsBacktracking
Decision tree exploration, pruning, permutations, combinations, subsets, N-queens.
325problems
Learn →Greedy
Make the locally optimal choice at each step. Interval scheduling, coin change, Huffman.
0problems
Learn →Graph
BFS, DFS, connected components, bipartite, DAGs, topological sort, SCC.
1,290problems
6 varsShortest Path
Dijkstra, Bellman-Ford, Floyd-Warshall, SPFA, 0-1 BFS, A*.
1,290problems
Learn →Bitmask DP
DP over subsets using bitmasks. TSP, assignment, graph covering, state space search.
4,313problems
Learn →Divide & Conquer
Master theorem, merge sort, quick sort, closest pair, maximum subarray (D&C).
1,290problems
Learn →Knapsack DP
0/1 knapsack, unbounded knapsack, bounded knapsack, subset sum, coin change II.
4,313problems
Learn →Graph - Advanced
Johnson's algorithm, minimum cost flow, Gomory-Hu tree, dominator tree.
1,290problems
Learn →Bipartite / Bicoloring
Check bipartite, maximum bipartite matching, König's theorem, Hall's theorem.
1,290problems
Learn →Coordinate Compression
Map sparse coordinates to dense indices for segment trees, BIT, DP.
4,313problems
Learn →Topological Sort
Kahn's algorithm, DFS-based sort, course schedule, dependency resolution.
1,290problems
Learn →Minimum Spanning Tree
Kruskal's, Prim's, Borůvka's, MST verification, second-best MST.
1,290problems
Learn →Network Flow
Ford-Fulkerson, Edmonds-Karp, Dinic's, max-flow min-cut, circulation.
1,290problems
Learn →Meet in the Middle
Split the input in half, solve each half, combine results. Subset sum, knapsack, TSP.
4,313problems
Learn →Convex Hull Trick
DP optimization: line container for max/min queries. Li Chao tree, dynamic CHT.
4,313problems
Learn →Centroid Decomposition
Decompose tree by centroids for O(log n) path queries. Distance queries, counting paths.
1,290problems
Learn →Line Sweep
Sweep-line algorithm for interval union, skyline, rectangle area, closest pair.
1,290problems
Learn →2-SAT
Implication graph, strongly connected components. Boolean satisfiability with 2 variables per clause.
1,290problems
Learn →Bridges & Articulation Points
Tarjan's algorithm for bridges, articulation points, biconnected components.
1,290problems
Learn →Min-Cost Max-Flow
Successive shortest augmenting path, potentials, cycle canceling, assignment problem.
1,290problems
Learn →Flow with Lower Bounds
Circulation with demands, feasible flow with lower/upper bounds, max flow with lower bounds.
1,290problems
Learn →Bidirectional BFS
Meet-in-the-middle on graphs. Word ladder, 8-puzzle, shortest path in large state spaces.
1,290problems
Learn →A* Search
Heuristic-guided search for shortest path. Manhattan distance, admissible heuristics.
1,290problems
Learn →Difference Constraints
System of inequalities reduced to shortest paths. Bellman-Ford on constraint graph.
1,290problems
Learn →Functional Graphs
Each node has exactly one outgoing edge. Cycle detection, distance to cycle, Josephus.
1,290problems
Learn →Matching via Flow
Bipartite matching via max flow. Assignment, Hall's marriage, vertex cover, edge cover.
1,290problems
Learn →Graph Coloring
Chromatic number, greedy coloring, Welsh-Powell, Brooks' theorem, interval graph coloring.
1,290problems
Learn →Top-K Streaming
Heavy hitters, count-min sketch, reservoir sampling, streaming median, frequent items.
1,290problems
Learn →Strongly Connected Components
Kosaraju's, Tarjan's algorithm. Condensation DAG, 2-SAT, dominator trees.
1,290problems
Learn →Graph State Space
Model problems as state transitions on a graph. Water jug, missionaries and cannibals, 8-puzzle.
1,290problems
Learn →Greedy Intervals
Interval scheduling, interval partitioning, meeting rooms, merge intervals, insert interval.
1,290problems
Learn →Constructive Algorithms
Build a valid solution step by step. Grid construction, permutation construction, array construction.
1,290problems
Learn →Backtracking with Pruning
Constraint propagation, forward checking, branch and bound, sudoku, cryptarithmetic.
1,290problems
Learn →Multi-Source BFS
Enqueue all sources at level 0. Rotten oranges, walls and gates, as far from land as possible.
1,290problems
Learn →Grid Islands
Number of islands, max area island, number of distinct islands, surrounded regions.
1,290problems
Learn →Part V - Strings, Sequences & Grid
String Algorithms
String fundamentals: reverse, rotation, comparison, pattern matching basics, anagram detection.
3,426problems
Learn →String Matching
KMP, Rabin-Karp, Z-algorithm. Find pattern in text, count occurrences, shortest palindrome.
3,426problems
Learn →Sequences
LIS, LCS, edit distance, longest arithmetic subsequence, wiggle sequence, Russian doll envelopes.
0problems
Learn →Matrix / Shape
Matrix traversal, rotation, spiral, set zeroes, word search, island perimeter, game of life.
331problems
Learn →String DP
DP on strings: edit distance, interleaving string, distinct subsequences, decode ways, regular expression.
4,313problems
Learn →Palindrome Patterns
Longest palindromic substring (Manacher's), count palindromes, palindrome partitioning, break a palindrome.
4,313problems
Learn →String Hashing
Rolling hash, Rabin-Karp, double hash, detect plagiarism, longest common substring via binary search + hash.
4,313problems
Learn →Suffix Array
Build suffix array, LCP array, longest repeated substring, number of distinct substrings, longest common substring.
4,313problems
Learn →Aho-Corasick
Multiple pattern matching automaton. Built on Trie + failure links. Find all occurrences of many patterns in text.
4,313problems
Learn →Z-Function
Compute Z-array in O(n): longest substring starting at i that matches prefix. String periodicity, pattern matching.
4,313problems
Learn →Manacher's Algorithm
Longest palindromic substring in O(n) using symmetry expansion and center caching.
4,313problems
Learn →Suffix Automaton
Minimal DFA accepting all suffixes of a string. Count distinct substrings, longest common substring, lexicographically smallest.
4,313problems
Learn →String Rotations
Lexicographically minimal rotation (Booth's), string matching on rotation, periodic strings.
4,313problems
Learn →Lyndon Factorization
Split string into Lyndon words (non-increasing). Duval's algorithm. Minimal rotation, necklace computation.
4,313problems
Learn →String Construction
Build smallest/greatest string under constraints. Lexicographically smallest after swaps, K-th lexicographically smallest.
4,313problems
Learn →String Window Patterns
Sliding window applied to strings: anagram substrings, min window substring, substring concatenation.
4,313problems
Learn →Part VI - Math & Discrete
Math
Number theory, combinatorics, Euclidean gcd/lcm, modular arithmetic, prime sieve.
9,632problems
Learn →Bit Manipulation
XOR tricks, bit hacks, Brian Kernighan, subset enumeration via bits, gray code, bitmask DP.
2,868problems
Learn →Combinatorics
Permutations, combinations, stars and bars, inclusion-exclusion, generating functions.
0problems
Learn →Game Theory
Nim, Grundy numbers, Sprague-Grundy theorem, impartial games, minimax.
0problems
Learn →Geometry
Convex hull, line intersection, polygon area, point in polygon, rotating calipers.
0problems
Learn →Number Theory
Extended Euclidean, modular inverse, CRT, Euler's totient, Fermat's little theorem, Miller-Rabin, Pollard's Rho.
9,632problems
Learn →Probability DP
Expected value DP, Markov chains, probability distributions, random walk, gambler's ruin.
4,313problems
Learn →Ternary Search
Find peak of unimodal function. Divide into three parts, discard one. Bitonic arrays, convex functions.
9,632problems
Learn →Fast Fourier Transform
Polynomial multiplication, convolution, big integer multiplication, signal processing.
9,632problems
Learn →Sprague-Grundy Theorem
Grundy numbers for impartial games. Compute mex, XOR combined games, game of Nim, Kayles, subtraction games.
9,632problems
Learn →Chinese Remainder Theorem
Solve system of linear congruences. Garner's algorithm for big integers. RSA-related problems.
9,632problems
Learn →Convex Hull
Graham scan, Andrew's monotone chain, Jarvis march, dynamic convex hull, convex hull of a polygon.
9,632problems
Learn →Inclusion-Exclusion
Count unions of sets, derangements, coprime count, principle applied to number theory and combinatorics.
9,632problems
Learn →Catalan Numbers
Counting: BSTs, parentheses, Dyck paths, triangulations, non-crossing partitions. Recurrence and closed form.
9,632problems
Learn →Josephus Problem
Find the last remaining position when every k-th is eliminated. O(n) DP, O(k log n) for large n.
9,632problems
Learn →Sieve Variants
Eratosthenes, linear sieve, segmented sieve, prime counting, divisor count, totient sieve, Mobius sieve.
9,632problems
Learn →Burnside's Lemma
Count distinct objects under group action. Orbit-counting, necklace/bracelet enumeration, Polya enumeration.
9,632problems
Learn →Gray Code
Binary representation where consecutive values differ by one bit. Sequence generation, n-queens bitset, subset generation.
9,632problems
Learn →Gaussian Elimination GF(2)
Solve linear systems over GF(2). XOR basis, linear basis of array, find if target reachable, maximum XOR subset.
9,632problems
Learn →Lucas' Theorem
Compute large binomial coefficients modulo prime. nCr mod p using base-p digits. Digit DP and combinatorics intersection.
9,632problems
Learn →Lattice Paths
Count/generate monotonic paths on integer lattice. Delannoy numbers, with/without obstacles, with/without diagonals.
9,632problems
Learn →Advanced Counting DP
Combinatorial DP beyond basics: Stirling numbers, Eulerian numbers, Bell numbers, partition DP, set partition.
9,632problems
Learn →Rotating Calipers
Width/diameter of convex polygon, minimum bounding rectangle, distance between convex polygons. O(n).
9,632problems
Learn →Carry DP
Digit DP where carry propagates through positions. Count numbers with sum of digits equal, etc.
9,632problems
Learn →Random Walk
Expected time to absorption, probability of reaching boundary, gambler's ruin, 1D/2D walk, drunkard's walk.
9,632problems
Learn →Number Tricks
Numerical properties, digital root, self numbers, happy numbers, narcissistic numbers, Kaprekar, Ulam, perfect numbers.
9,632problems
Learn →Part VII - Advanced Topics
Linear Algebra
Matrix operations, Gaussian elimination, determinant, rank, eigenvalues, linear transformations.
0problems
Learn →Numerical Methods
Newton's method, binary search for roots, integration, differentiation, gradient descent, Monte Carlo.
0problems
Learn →Scheduling
Job sequencing, interval scheduling, CPU scheduling, task scheduling with dependencies, resource allocation.
0problems
Learn →Shapes / Geometry
Area/perimeter/volume of geometric shapes, union/intersection of shapes, packing, tiling, cutting stock.
331problems
Learn →Miscellaneous
Cache policies (LRU, LFU, ARC), OOP design, system design, concurrency problems, brainteasers.
0problems
Learn →Design Patterns
Singleton, factory, observer, strategy, decorator, adapter, command, iterator. OOP design in coding interviews.
0problems
Learn →Sorting Algorithms
Quick sort, merge sort, heap sort, counting sort, radix sort, bucket sort, tim sort, intro sort.
4,313problems
Learn →Simulation
Implement the problem description faithfully. Text processing, game simulation, protocol implementation.
0problems
Learn →Matrix Exponentiation
Linear recurrences via fast matrix power. Fibonacci in O(log n), linear DP acceleration.
9,632problems
Learn →Randomized Algorithms
Randomized quickselect, reservoir sampling, Fisher-Yates shuffle, Monte Carlo methods, Miller-Rabin.
4,313problems
Learn →Multiset & Ordered Set
Order statistic tree, sorted container, balanced BST simulation. K-th element, count of elements in range.
4,313problems
Learn →Part VIII - Cross-Topic Deep Dives
BFS vs DFS
When BFS beats DFS and vice versa. Shortest path, topological, bipartite, connected components, backtracking.
1,290problems
Learn →Interval Problems
Interval union, intersection, overlap, covering, partition, scheduling. Sweep-line, greedy, segment tree.
0problems
Learn →DP on Trees
Tree DP: diameter, paths, subtree aggregates, rerooting DP, knapsack on tree, tree matching.
2,714problems
Learn →Union-Find (DSU)
Disjoint set union with path compression and union by size. Dynamic connectivity, Kruskal, connected components.
1,290problems
Learn →Digit DP
DP on number digits with tight/loose bounds. Count numbers with property in range. Sum of digits, divisible by k.
4,313problems
Learn →Interval DP
DP over intervals: matrix chain multiplication, burst balloons, palindrome partitioning II, stone game, optimal BST.
4,313problems
Learn →Grid DP
DP on 2D grids: unique paths, minimum path sum, dungeon game, cherry pickup, triangle, falling path sum.
4,313problems
Learn →State Machine DP
DP with explicit states. Best time to buy/sell stock, paint house, student attendance, k transactions.
4,313problems
Learn →Recursion + Memoization
Top-down DP, memoization patterns, recursion tree optimization, stack depth management, tabulation conversion.
4,313problems
Learn →Grid Patterns
Neighbor iteration, matrix prefix sum, BFS/DFS on grid, island patterns, shortest distance, multi-source.
1,290problems
Learn →Fenwick Tree (BIT)
Range sum/product queries with point updates. LIS count, inversion count, order statistics, offline queries.
4,313problems
Learn →Heavy-Light Decomposition
Decompose tree into heavy/light paths for path queries with segment tree. Path sum, max, min, update.
1,290problems
Learn →Euler Tour (ETT)
Flatten tree into array via Euler tour. Subtree queries become range queries. LCA via RMQ, subtree aggregates.
1,290problems
Learn →Mo's Algorithm
Offline query processing with sqrt decomposition. Sort queries by block, maintain pointer movement. Range queries.
4,313problems
Learn →XOR Basis / Linear Basis
Basis of numbers under XOR. Maximum XOR subset, minimum XOR subset, K-th smallest XOR, rank of XOR space.
4,313problems
Learn →Euler Path / Circuit
Hierholzer's algorithm for Eulerian path. Chinese postman problem, de Bruijn sequence. Hierholzer, Fleury.
1,290problems
Learn →Divide & Conquer DP
DP optimization: monotone opt property (quadrangle inequality). DP[i][j] = min over k < j of DP[i-1][k] + C[k][j].
4,313problems
Learn →Square Root Decomposition
Partition array into sqrt blocks. Batch queries, lazy rebuild. Range sum/update, mode query, k-th smallest in range.
4,313problems
Learn →Persistent Segment Tree
Versioned segment trees. K-th smallest in range, count distinct in range, sum/update across versions.
4,313problems
Learn →Treap (Randomized BST)
Randomized BST with heap property. Split/merge operations. Range reverse, lazy propagation, order statistics, implicit treap.
4,313problems
Learn →Offline CDQ Divide & Conquer
CDQ divide-and-conquer for multi-dimensional offline queries. 3D partial order, dynamic convex hull, DP optimization.
4,313problems
Learn →Bitset Operations
std::bitset / BigInt manipulation for DP optimization. Bitset knapsack, graph reachability, matching, subset queries.
4,313problems
Learn →Slope Trick
DP optimization for convex piecewise-linear functions. Priority queue management. Tree DP, grid DP, scheduling DP.
4,313problems
Learn →Segment Tree Beats
Segment tree with min/max operations that break standard lazy. Range chmin/chmax, range sum queries in O(n log n).
4,313problems
Learn →Balanced DP
DP with equilibrium constraints. Balance brackets, load balancing, fair division, partition with balance target.
4,313problems
Learn →Profile DP (DP on Broken Profile)
DP over row/column with state profile. Bitmask DP on grids, tiling problems, crosswords, domino tilings.
4,313problems
Learn →Aliens Trick (Lagrange DP)
DP optimization using Lagrangian relaxation. Attach penalty lambda for constraint, binary search lambda. k-trades, partition.
4,313problems
Learn →Arithmetic DP
DP on arithmetic properties. Sum of digits, divisibility, carry DP, digit DP variants with arithmetic constraints.
4,313problems
Learn →Subsequence Counting
Count distinct subsequences, number of subsequences with given sum/product, subsequence pattern matching.
4,313problems
Learn →Sliding Window - Advanced
At-most-k trick, exactly-k count formula, multiple-pointer windows, deque optimization for range max/min.
4,313problems
Learn →2D Prefix Sum
Prefix sum on 2D grid. Range sum queries, submatrix sum, count submatrices with sum ≤ k.
4,313problems
Learn →DP with Deque Optimization
Sliding window DP optimization: DP[i] = max over j in window of DP[j] + f(j,i). Range min/max of DP values.
4,313problems
Learn →Stock Trading
Best time to buy/sell with 1, 2, k transactions. Cooldown, transaction fee, frozen state. State machine DP.
4,313problems
Learn →DP Space Optimization
Rolling array, 1D to O(1) space, Knuth optimization, monotone queue optimization, divide & conquer optimization.
4,313problems
Learn →String Window Patterns
Sliding window applied to strings: anagram substrings, min window substring, substring concatenation, find all anagrams.
4,313problems
Learn →