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Foundations

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  1. BASICSBeginnerContinue →FreeFoundations - Big-O NotationThe language every engineer uses to describe speed - what O(n), O(log n), and friends actually measure, and why constants stop mattering.10m
  2. BASICSBeginnerFreeFoundations - Logarithms for EngineersA logarithm asks 'how many times must I halve (or double)?' - the math hiding inside binary search, balanced trees, and every O(log n) promise.8m
  3. BASICSBeginnerMembersFoundations - Binary Numbers & Bitwise OperationsComputers count in ones and zeros - learn to read binary, meet the bitwise operators, and understand why powers of two rule memory.9m
  4. BASICSBeginnerMembersFoundations - Arrays & MemoryArrays live side by side in memory - which is why indexing is instant, inserting in the middle is not, and caches love them.7m
  5. BASICSBeginnerMembersFoundations - Recursion & the Call StackA function that calls itself sounds scary until you see the call stack - master base cases and you unlock trees, sorting, and DP.8m
  6. BASICSBeginnerMembersFoundations - Sets & Maps (Hash Intuition)Ask 'have I seen this before?' or 'what do I know about X?' in one step - hash sets and maps are the most-used tools in interviews.7m
  7. BASICSBeginnerMembersFoundations - Graphs & NetworksNodes and edges model everything from friendships to road maps - learn the vocabulary before touching graph algorithms.7m
  8. BASICSBeginnerMembersFoundations - Statistics Engineers Actually UseMean vs median, percentiles, and expected value - the numbers behind latency dashboards and 'why averages lie'.7m
  9. BASICSBeginnerMembersFoundations - Complexity Classes ComparedWatch every complexity class race real input sizes - and learn where the practical limits of computing actually sit.7m
  10. BASICSBeginnerMembersFoundations - Reading Code Traces on This SiteLearn to read the variable-table trace format used across this platform - pointers, highlights, and invariants in five minutes.6m

Data Structures

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  1. DSBeginnerContinue →FreeLinked List - Insert at HeadO(1) insertion. Allocate a node, rewire next, and move the head pointer.5m
  2. DSIntermediateFreeLRU CacheHash map for O(1) lookup plus a doubly linked list for recency order. Least-recently-used evicts first.9m
  3. DSBeginnerMembersValid ParenthesesCheck that every bracket is closed in the right order using a stack.6m
  4. DSBeginnerMembersMerge Two Sorted ListsMerge two sorted lists into one by always taking the smaller next node.7m
  5. DSBeginnerMembersReverse Linked ListReverse a linked list in place by rewiring each node's next pointer with three pointers.7m
  6. DSBeginnerMembersMaximum Depth of Binary TreeFind how deep a binary tree goes by computing subtree heights bottom-up.6m
  7. DSBeginnerMembersInvert Binary TreeMirror a binary tree by swapping each node's left and right children.6m
  8. DSIntermediateMembersValidate Binary Search TreeCheck a BST's ordering with an in-order walk or a value range.8m
  9. DSIntermediateMembersBinary Tree Level Order TraversalVisit a tree row by row using a queue - breadth-first search.7m
  10. DSBeginnerMembersLinked List CycleDetect a cycle in a linked list with Floyd's slow and fast pointers.7m
  11. DSAdvancedMembersBinary Tree Maximum Path SumFind the largest sum of any path in a binary tree with a post-order gain function.10m
  12. DSIntermediateMembersLowest Common Ancestor of a BSTFind the deepest node that has both targets as descendants using the BST order.8m
  13. DSAdvancedMembersSerialize and Deserialize Binary TreeEncode a tree to a string and rebuild it exactly, using pre-order with null markers.10m
  14. DSBeginnerMembersSame TreeCheck whether two binary trees are identical with a parallel recursive walk.6m
  15. DSIntermediateMembersData Structure - Heap & Priority QueueA binary heap gives O(1) access to the min/max and O(log n) insert - the engine behind top-k, schedulers, and Dijkstra.9m
  16. DSIntermediateMembersData Structure - Trie (Prefix Tree)Store words character by character so prefix lookups cost O(length), not O(dictionary size) - the engine of autocomplete.8m

Algorithms

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  1. ALGOBeginnerContinue →FreeBinary SearchHalve the search space every step. Trace low, mid, and high as they converge on the target.6m
  2. ALGOBeginnerMembersBubble SortThe largest unsorted value bubbles to the end of each pass. Watch swaps happen in place.8m
  3. ALGOBeginnerFreeTwo SumFind two numbers that add up to a target. The single most-asked interview question.6m
  4. ALGOBeginnerMembersValid PalindromeCheck whether a string reads the same forward and backward with two pointers.5m
  5. ALGOBeginnerMembersValid AnagramDecide whether two strings are the same letters rearranged, by counting characters.5m
  6. ALGOBeginnerMembersContains DuplicateDetect whether an array has any repeated value using a hash set.4m
  7. ALGOBeginnerMembersBest Time to Buy and Sell StockFind the maximum profit from one buy and one sell by tracking the lowest price seen so far.6m
  8. ALGOIntermediateMembersMaximum SubarrayFind the contiguous subarray with the largest sum using Kadane's algorithm.8m
  9. ALGOIntermediateMembersNumber of IslandsCount connected land regions in a grid with a flood-fill DFS.9m
  10. ALGOBeginnerMembersClimbing StairsCount ways to climb n stairs taking 1 or 2 steps - the Fibonacci recurrence.7m
  11. ALGOIntermediateMembersHouse RobberMax money you can rob from a row of houses without hitting adjacent ones.8m
  12. ALGOIntermediateMembers3SumFind all triplets that sum to zero using sort plus two pointers.10m
  13. ALGOIntermediateMembersProduct of Array Except SelfReturn the product of every element except the current one, without division.8m
  14. ALGOIntermediateMembersCoin ChangeFind the fewest coins that make a target amount with unbounded denominations.10m
  15. ALGOIntermediateMembersGroup AnagramsGroup words that are anagrams using a sorted string as the key.8m
  16. ALGOIntermediateMembersLongest Substring Without Repeating CharactersFind the longest substring with all distinct characters using a sliding window.8m
  17. ALGOIntermediateMembersTop K Frequent ElementsReturn the k most common values using bucket sort by frequency.8m
  18. ALGOIntermediateMembersWord BreakDecide if a string can be split into dictionary words with DP.10m
  19. ALGOIntermediateMembersDecode WaysCount how many ways a digit string can decode into letters with a Fibonacci-style DP.9m
  20. ALGOIntermediateMembersCourse ScheduleDecide if all courses can be taken by detecting a cycle with Kahn's topological sort.10m
  21. ALGOBeginnerMembersNumber of 1 BitsCount set bits in an integer by clearing the lowest 1 with n & (n - 1).6m
  22. ALGOBeginnerMembersMissing NumberFind the missing number from 0..n using the sum formula or XOR.6m
  23. ALGOIntermediateMembersRotate ImageRotate an n x n matrix 90 degrees clockwise in place via transpose and reverse.8m
  24. ALGOIntermediateMembersMerge IntervalsMerge overlapping intervals by sorting on start, then fusing overlaps in one pass.8m
  25. ALGOIntermediateMembersInsert IntervalInsert a new interval into a sorted non-overlapping list and merge any overlaps.8m
  26. ALGOBeginnerMembersMeeting RoomsDecide if one person can attend all meetings by checking sorted intervals for overlaps.6m
  27. ALGOIntermediateMembersAlgorithm - Union-Find (Disjoint Set Union)Track which items belong to the same group in near-constant time - the secret weapon behind cycle detection and connected components.8m
  28. ALGOAdvancedMembersAlgorithm - Dijkstra's Shortest PathGreedy + priority queue settles the closest unfinalized node each round - shortest paths for any graph with non-negative weights.10m
  29. ALGOBeginnerMembersAlgorithm - Merge SortDivide in half, sort recursively, merge linearly - the stable O(n log n) guarantee and the gateway to external sorting.7m
  30. ALGOBeginnerMembersAlgorithm - Quick SortPartition around a pivot and recurse - the fastest in-memory sort in practice, with an O(n²) worst case you must know how to avoid.8m

System Design

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  1. SYSIntermediateContinue →FreeSystem Design - URL ShortenerA short URL service end to end: short-code generation, storage, caching, and redirects.7m
  2. SYSBeginnerMembersSystem Design - How DNS WorksTurn a name into an IP: recursive resolution from the resolver to the root, TLD, and authoritative servers.6m
  3. SYSIntermediateFreeSystem Design - CachingServe hot data from memory and pick the write strategy that matches your risk tolerance.9m
  4. SYSIntermediateMembersSystem Design - CAP TheoremA distributed system can guarantee only two of three: consistency, availability, and partition tolerance.7m
  5. SYSIntermediateMembersSystem Design - SQL vs NoSQLPick the store that fits the data shape: relational for joins and ACID, NoSQL for scale and flexibility.8m
  6. SYSAdvancedMembersSystem Design - Scaling to Millions on AWSGrow a system from one box to millions of users by removing the bottleneck at each step.12m
  7. SYSAdvancedMembersSystem Design - Twitter Timeline and SearchPost a tweet, fan it out to follower timelines, serve reads from cache, and search an index.12m
  8. SYSIntermediateMembersSystem Design - Content Delivery NetworkServe content from servers near the user so it loads fast anywhere in the world.8m
  9. SYSIntermediateMembersSystem Design - Load BalancerSpread traffic across servers so no single machine is overwhelmed, and hide failures.8m
  10. SYSIntermediateMembersSystem Design - Reverse ProxyA server that sits in front of your backend, forwards requests, and shields it from clients.7m
  11. SYSIntermediateMembersSystem Design - Application Layer and MicroservicesSplit an app into small independent services, each owning one job and its own data.9m
  12. SYSAdvancedMembersSystem Design - Database ScalingKeep the database fast as it grows: replication, federation, sharding, and denormalization.11m
  13. SYSIntermediateMembersSystem Design - Consistency PatternsWeak, eventual, or strong: how fresh a read is, and what you pay for it.8m
  14. SYSIntermediateMembersSystem Design - Availability PatternsFailover and replication keep a system up - and the nines measure how well.8m
  15. SYSIntermediateMembersSystem Design - Asynchronism and QueuesDo slow work later, off the request path, so the user is never kept waiting.8m
  16. SYSIntermediateMembersSystem Design - Communication ProtocolsHow services talk: TCP vs UDP underneath, and RPC vs REST on top.9m
  17. SYSIntermediateMembersSystem Design - Security and TLSTLS encrypts traffic end to end: a handshake exchanges keys, then symmetric crypto seals the stream.7m
  18. SYSAdvancedMembersSystem Design - Web CrawlerDownload the web page by page: a frontier queue, polite fetching, dedup, and an index.12m
  19. SYSIntermediateMembersSystem Design - Personal Finance AggregatorPull transactions from many banks, normalize them, and categorize spending.10m
  20. SYSAdvancedMembersSystem Design - Social GraphModel users and follows as a graph, then traverse it for suggestions and feeds.10m
  21. SYSAdvancedMembersSystem Design - Key-Value StoreConsistent hashing maps keys to shards so adding a node moves few keys, with quorums for consistency.11m
  22. SYSAdvancedMembersSystem Design - Sales RankingTurn a stream of orders into a live bestseller list with incremental counters and a top-N computation.10m
  23. SYSIntermediateMembersOO Design - Parking LotModel a parking lot: levels, spots, vehicles, and tickets, with O(1) spot lookup.9m
  24. SYSBeginnerMembersOO Design - Deck of CardsModel cards, a deck, hands, and a game - and shuffle correctly with Fisher-Yates.7m
  25. SYSIntermediateMembersOO Design - Call CenterRoute calls to the first available employee, then escalate up the chain of command.8m
  26. SYSAdvancedMembersOO Design - Online ChatModel users, conversations, and messages, with fan-out delivery and presence tracking.9m
  27. SYSIntermediateMembersSystem Design - Back-of-Envelope EstimationEstimate QPS, storage, and latency in your head to sanity-check any design before building.8m
  28. SYSIntermediateMembersSystem Design - Rate LimiterReject excess traffic with counters and windows - fixed, sliding, and token-bucket strategies compared.9m
  29. SYSIntermediateMembersSystem Design - Distributed Message QueueDecouple producers from consumers with a durable log - offsets, delivery guarantees, and ordering trade-offs.10m
  30. SYSIntermediateMembersSystem Design - Proximity Service (Geo-Indexing)Find everything nearby at city scale - geohash grids, cell rings, and the write-heavy reality of moving objects.10m
  31. SYSAdvancedMembersSystem Design - Undo/Redo & Collaborative EditingCommand stacks for solo undo, then OT and CRDT strategies when many cursors edit one document at once.10m
  32. SYSIntermediateMembersSystem Design - Notification SystemFan one event out to millions across push, email, and in-app - with preferences, dedupe, and rate caps at every hop.9m