+1URL router with single-segment wildcardsCodingBuild a small URL router. - A pattern and a path are split on / into segments. Empty segments are ignored, so /a/b/ is the same as /a/b, and / is the empty path. - A p
Last asked Oct 2026
#design
- +1
Per-user chat event counts in a sliding windowCodingA chat service runs for months without restarting. It sends you a stream of message events. Each event has a userid, a chatid and an integer timestamp in seconds. You must answer…Last asked Oct 2026
+4Anthropic OA: leveled in-memory record storeCodingA timed, multi-level exercise building an in-memory store of records with fields: create and read first, then scans and filters, then timestamps with expiry, then backup and…Last asked Oct 2026- Fault-tolerant work queue with leases and retriesCodingImplement an in-memory work queue that keeps working when workers crash. Workers take a task, process it, and report success or failure. A worker that dies silently must not lose…Last asked Oct 2026
Maximum frequency stackCodingBuild a stack of integers with push(x) and pop(). pop removes and returns the value that occurs most often in the stack; when several values tie, it takes the one with an…Last asked Oct 2026
Route requests to the nearest healthy data centerCodingBuild a router that processes commands: register a data center with its latitude, longitude and capacity, mark data centers up or down, report the great-circle distance between…Last asked Sep 2026
+1Connect Four game engine with win detectionCodingImplement a two-player game of Connect Four. The board has rows rows and cols columns (default 6 x 7). Players "A" and "B" alternate turns, and "A" moves first. On a turn, a…Last asked Sep 2026- +1GPU credit ledger with out-of-order eventsCodingBuild an in-memory ledger for GPU credits. Credit arrives as grants: each one has a unique id, an amount, and a time window in which it can be spent. Grants may overlap in time…Last asked Sep 2026
Weighted cache that evicts the heaviest entriesCodingImplement WeightedCache, a key-value cache with a total-weight budget instead of an item count. Each entry has a positive integer weight. After every put, the sum of the weights…Last asked Sep 2026- +2Banking system with transfers, cashback and account mergesCodingBuild an in-memory banking system in four levels. Each level adds methods, and every earlier method must keep working. Every call takes a timestamp in milliseconds as its first…Last asked Sep 2026
- +1Social follow graph with point-in-time snapshotsCodingBuild an in-memory follow graph that can answer questions about its own past. Follows are directed: a following b says nothing about b following a. Implement a SocialNetwork…Last asked Sep 2026
- Text editor in stages: buffer, undo and redo, autocomplete, collaborationCodingBuild the core of a text editor in four stages, each on top of the last, so keep the design easy to extend: 1. A text buffer with insert and delete at a position. 2. Undo and redo…Last asked Sep 2026
- Event counter over a trailing 15-minute window with a ring bufferCodingBuild an event counter for a monitoring system with two operations: record(timestamp) notes that an event happened at a given second, and count(now) returns how many events…Last asked Sep 2026
+1Generalized tic-tac-toe with k in a rowCodingDesign a two-player Tic-Tac-Toe game on a board with n rows and m columns. A player wins by placing k of their own marks consecutively in one line: horizontal, vertical, diagonal…Last asked Sep 2026- +1Customer revenue with referral credit and bottom-k queryCodingBuild an in-memory system that tracks how much revenue each customer brings in, including revenue from the customers they referred. Customers get integer ids starting at 0, in the…Last asked Sep 2026
Shopping cart with discounts, undo and redoCodingBuild a shopping cart with item discounts where every mutation supports undo and redo. Reported on 1point3acres' Decagon bank; reconstructed in our own words.Last asked Sep 2026- Key-value store that reports its recent request rateCodingBuild an in-memory key-value store with put and get (delete optional). It must also report how many puts and gets per second it served over a recent stretch of time, say five…Last asked Sep 2026
- Ordered IP firewall with CIDR rulesCodingImplement an IPv4 firewall that checks addresses against an ordered list of rules. Each rule is a pair (action, target). action is "ALLOW" or "DENY". target is either a single…Last asked Sep 2026
+5Sliding-window request rate limiterCodingImplement a rate limiter that accepts at most limit requests in any window of ttl milliseconds. - allow(timestamp) is called once per incoming request. timestamp is in…Last asked Sep 2026- +2Rate-limited event loggerCodingBuild a logger that rate-limits repeated events in a stream. Each event has a string name and an integer timestamp in seconds. Timestamps arrive in non-decreasing order, and…Last asked Sep 2026
- +1Authentication token manager with expiryCodingBuild a manager for tokens that live for a fixed time: generate(id, now), renew(id, now) which only works on a token that has not expired, and countunexpired(now). Discuss how…Last asked Sep 2026
- +2Cloud storage with user quotas and backupsCodingDesign an in-memory cloud storage service. The interview has four parts. Each part adds methods, and every earlier method must keep working. Implement a class CloudStorage. File…Last asked Sep 2026
- Most active user by distinct contacts, then top KCodingUsers send each other messages, and a message between A and B counts as contact for both of them. Build a class with: - register(a, b): record a message between a and b. -…Last asked Sep 2026
+8Temporal key-value store with point-in-time reads and restoreCodingBuild an in-memory key-value store that keeps every version of every key, so you can ask what a key held at any point in time. Implement the TemporalKVStore class: -…Last asked Sep 2026- Predict the most frequent next word in constant timeCodingFrom a body of text, build a predictor that, given a word, returns the word that most often came right after it. Each prediction must take O(1) time. Discuss how you keep that…Last asked Sep 2026
- Duplicate detection: sliding windows, then read-heavy queriesCodingStart with checking an array for duplicate values, then evolve the solution step by step: first find duplicates that fall within a sliding window of positions, then rework the…Last asked Sep 2026
- Linked-list queue with deletionsCodingImplement a queue on a linked list with enqueue and dequeue, then extend it through follow-ups that delete elements from the queue.Last asked Sep 2026
- +1Oldest one-time visitor in a login streamCodingAn online store wants to know which customers have visited only once. Every login is reported to your service as a username, in the order the logins happen. Implement a class…Last asked Aug 2026
- Train fare calculator with a weekend capCodingA train line runs through stations G, U, H, K and I in that order. Each hop between neighbouring stations costs 1, so a trip costs the number of hops it covers (G to I costs 4)…Last asked Aug 2026
Hit counter for the last five minutesCodingRecord hits (or events) as they arrive with a timestamp, and answer how many happened in the last five minutes (more generally, the last W seconds) at any moment. Discuss which…Last asked Aug 2026- Hybrid run-length and bit-packing codecCodingDesign an encoder and a decoder for a list of non-negative integers. The encoder compresses the list into a list of human-readable run strings. The decoder restores the original…Last asked Aug 2026
Rate limiter with multiple sliding-window rulesCodingBuild a rate limiter that enforces several rules at once. Each rule is a pair (limit, window): at most limit accepted requests may fall in any window of window time units. Use the…Last asked Aug 2026- Track missing package IDs from out-of-order arrivalsCodingPackages with IDs 1, 2, 3, ... arrive in any order. Support recording an arrival, reporting the contiguous frontier (the largest ID such that every ID up to it has arrived), and…Last asked Aug 2026
- Indexed sequence with checkpointsCodingBuild a growable array that can take cheap snapshots. The spec arrives in three parts; finish and test each part before the next. Part 1. append(value) - index, pop() - value…Last asked Jul 2026
- Flatten a 2D array with removeCodingImplement a class Vector2D that walks a 2D list of integers row by row, left to right, and lets the caller delete values while walking. text Vector2D(vec: listlistint) rows may be…Last asked Jul 2026
- Threshold-based database partition rebalancerCodingWrite a rebalancer for database partitions spread across nodes: when a node's load goes over a threshold, move partitions off it onto other nodes. Explain which partitions you…Last asked Jul 2026
- Recipe shopping cart with bulk discountsCodingModel a recipe shopping cart that prices ingredients with bulk discounts per ingredient. Reported on 1point3acres' Decagon bank; reconstructed in our own words.Last asked Jul 2026
- Cart with nested transactionsCodingImplement a cart whose updates run inside nestable transactions that commit or roll back as a unit. Reported on 1point3acres' Decagon bank; reconstructed in our own words.Last asked Jul 2026
+1Design layer document with undo, redo and batchesCodingYou are building the core of a design tool. A document holds layers. Each layer has a unique integer id and a set of string properties, such as color, shape or size. A layer does…Last asked Jul 2026- Range moduleCodingTrack half-open integer ranges left, right): add a range, remove a range, and ask whether every number in a range is currently tracked.Last asked Jul 2026
Transactional in-memory key-value databaseCodingBuild an in-memory key-value store with transactional reads and writes: operations inside a transaction see each other's effects and commit or roll back atomically. Reported on…Last asked Jul 2026- Bank ledger with two-step transfers and mergesCodingModel simple bank accounts with deposits and withdrawals, a transfer flow that holds funds until the recipient accepts, and an account-merge step that must keep working even if a…Last asked Jun 2026
- Encode and decode TinyURLCodingImplement a class that turns a long URL into a short one and back, in memory. Discuss counter-based versus random or hash-based codes, collisions, the same long URL submitted…Last asked Jun 2026
- Which interval contains a number?CodingBuild a class with addNonOverlappingInterval(start, end) and a query that says whether a number falls inside a stored interval (and which one). Discuss three versions: a linear…Last asked Jun 2026
Todo list system design in codeCodingDesign and implement a small todo list system with lists, tasks and completion state. Reported on 1point3acres' Notion bank; reconstructed in our own words.Last asked Jun 2026- Filter events and route them to queuesCodingProcess a stream of events: apply filter rules to each one and route every event that passes to the right queue.Last asked May 2026
- Agent to-do list with task dependenciesCodingBuild the task tracker an AI agent uses to plan its work. The question comes in three parts. Each part builds on the one before, so keep earlier behavior working. A status is an…Last asked May 2026
- Simplified filesystem with paths and traversalCodingImplement a small filesystem with path lookup and directory traversal, extended in the final level with model-related behavior. The harness is strict, so edge cases around paths…Last asked May 2026
- Stepwise DNS resolver with aliasingCodingBuild a simplified DNS resolver step by step against a fixed test harness: basic name lookup first, then alias layers on top. Later steps assume the earlier ones work. - Level 1:…Last asked May 2026
- Group booking across properties in one neighborhoodCodingA rental company houses groups in its properties. A group may be split across several properties, but all of them must be in the neighborhood the group asked for. Implement…Last asked May 2026
+3Key-value store with nested transaction rollbackCodingImplement an in-memory key-value store whose string keys map to string values. It supports transactions, and transactions can be nested. Implement the class KeyValueStore: -…Last asked May 2026- Searchable sentence indexCodingBuild an index over a collection of sentences so you can quickly find the sentences that contain given words.Last asked May 2026
- +1Live top-k best-selling books leaderboardCodingA bookstore wants a live leaderboard of its best-selling books. Sales arrive one at a time, and the leaderboard can be read at any moment. Implement a BookSalesTracker class: -…Last asked May 2026
Referral networkCodingImplement a referral network: users refer other users, and the network records who referred whom. Support adding referrals and answering questions about a user's referrals, such…Last asked May 2026- Set over a bounded range of integersCodingImplement a set of integers drawn from a known bounded range, with add, remove and contains.Last asked Apr 2026
Key-value store with constant-time set-allCodingImplement a key-value store with set, get and a setAll operation that assigns one value to every key, all in O(1). A related variant manages dataset sizes under a fixed disk…Last asked Apr 2026- Lazy array with deferred map and indexOfCodingImplement LazyArray, an integer array that supports deferred mapping. Calls to map are recorded but do not run. The recorded functions run only when a query needs a value…Last asked Apr 2026
- Ads retrieval service using a heapCodingBuild a service that returns the best ads for a request, ranked by score, using a heap. Support adding and removing ads, and discuss the cost of each operation.Last asked Mar 2026
+1Single-threaded scheduler for one-shot, delayed and periodic tasksCodingBuild a scheduler that runs on a single thread and accepts three kinds of jobs: run once now, run once after a delay, and run every N units of time. Decide how the clock moves…Last asked Feb 2026
Range sum query (immutable)CodingPreprocess an integer array so that the sum of any range i, j is answered in constant time. Build prefix sums once and design the function so it can be reused across many queries.Last asked Feb 2026- +1Pluggable hierarchical rate limiterCodingDesign a rate limiter with hierarchical limits (user, team, global) whose limit rules are pluggable without changing the core. Reported on 1point3acres' Cursor bank; reconstructed…Last asked Feb 2026
- Worker hours registerCodingImplement a register that records the hours workers log and answers questions about each worker's totals.Last asked Feb 2026
- Kubernetes service dependency queriesCodingYou get a list of Kubernetes services, each with attributes, a status and the services it depends on. Build a structure over them and answer queries such as which services a…Last asked Feb 2026
- +1LRU cache with runtime resizingCodingImplement a least recently used (LRU) cache whose capacity can change while it is in use. - get(key) returns the value for key, or -1 if it is not cached. A successful get makes…Last asked Feb 2026
Cache with a snapshot printerCodingBuild a cache much like an LRU cache, with one extra function, snapshot(cache), that prints the cache's current contents in order. A second part builds on it with a compressed…Last asked Feb 2026- Moving average from a data streamCodingImplement a class that receives numbers one at a time and returns the average of the last size values after each one, in constant time per call, keeping a running sum and a queue…Last asked Jan 2026
- Radix cache for integer sequences with LRU evictionCodingImplement RadixCache, a cache of integer sequences (for example, token IDs of model prompts) stored in a radix tree: a trie whose edges hold runs of integers instead of single…Last asked Jan 2026
- Iterator that yields one array first, then the rest of anotherCodingDesign an iterator over two integer arrays, a and b, that returns elements in this order: 1. Every element of b, in its original order (duplicates included). 2. Then every element…Last asked Jan 2026
- Counter class with add, count and removeCodingWrite a class that stores strings with how many times each was added. Support add(s), count(s) and remove(s). Follow-ups: return the total count across everything stored, and how…Last asked Sep 2025
- Sum tree with node values computed from childrenCodingA tree's leaves hold numbers, and every other node's value is the sum of its children. Return the value of any node. Then support changes: turn a node into a leaf with a value, or…Last asked Sep 2025
Cache with expiry, priority and LRU evictionCodingImplement a fixed-capacity key-value cache, PriorityExpiryLRUCache. Every item stores a value, an integer priority, and an expiry time. When a new key arrives and the cache is…Last asked Aug 2025- Lock, unlock and upgrade nodes in a treeCodingA tree is given by a parent array. Implement lock(node, user), unlock(node, user) and upgrade(node, user): a node can be locked by a user if it is unlocked; unlocked only by the…Last asked Aug 2025
- Key-value map with snapshotsCodingBuild a key-value map that supports set, get and snapshot. snapshot() returns an ID, and you can then read what any key held as of that snapshot. Avoid copying the whole map at…Last asked Aug 2025
- Insert delete getrandom O(1)CodingDesign a set with insert, remove and getRandom (each element equally likely), all in average constant time, using an array of values and a map from value to index; remove by…Last asked Jul 2025
- Max stackCodingDesign a stack that also supports peekMax() and popMax() (removing the top-most copy of the largest value), along with push, pop and top. Compare a two-stack design with a doubly…Last asked Jul 2025
- Alert routing service with silences and escalationCodingBuild the routing core of an alerting service. The monitoring system calls fire(labels, now) for an active alert on every evaluation cycle (every 30 seconds or so) and…
Read-through and write-through cache adapterCodingImplement a cache adapter that reads through to the backing store on misses and writes through on updates. Reported on 1point3acres' Harvey bank; reconstructed in our own words.
Access log storage with time range queriesCodingStore user access log entries and answer queries for entries inside a time range efficiently. Reported on 1point3acres' Glean bank; reconstructed in our own words.- Recipe storage with version historyCodingStore recipes with full version history: every edit creates a retrievable version and any past version can be read back. Reported on 1point3acres' Glean bank; reconstructed in our…
Ordered integer streamCodingMaintain an integer stream that yields values in order as out-of-order inserts arrive. Reported on 1point3acres' Sierra bank; reconstructed in our own words.- Page permission managementCodingImplement page-level permissions: grant and revoke access for users on pages, and answer whether a user can view or edit a page, respecting the page hierarchy. Reported on…
- Disk block writes with limited write cyclesCodingA storage device has n blocks with ids 0 to n - 1. Every block can be written at most maxcycles times; after that it is worn out and can never be written again. Implement a…
- +1Recent search history with a size limitCodingBuild the recent-searches list behind a search box. When the user focuses the box, it shows their most recent distinct searches, newest first. Every search the user runs is…
- Queue with constant-time min and maxCodingImplement a FIFO queue of integers that can also report its smallest and largest element at any time. Implement the MinMaxQueue class: - MinMaxQueue() creates an empty queue. -…
- Key-value store with prefix searchCodingBuild an in-memory key-value store whose values can also be looked up by key prefix. Keys are strings and values are integers. Implement the KVStore class: - KVStore() creates an…
- IP routing table with longest prefix matchCodingBuild the forwarding table of an IPv4 router. Routes are CIDR prefixes mapped to a next-hop name. When a packet arrives, the router must send it to the next hop of the most…
- Recent file tracker with date and insertion orderCodingImplement a FileStorageSystem class that records files and answers two kinds of listing queries. - storefile(file: str, date: str) - None records a file with its storage date…
- Connect points that share a row or columnCodingDesign a structure over an unbounded 2D grid. Each cell is either empty or holds a point. You can move from one point to another point in one step when the two points share a row…
- Per-domain sliding-window rate limiterCodingA gateway receives requests one at a time, at most one per second. Each request names a domain. Build a rate limiter that keeps its state between calls and decides, for each…