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Stateful API
  • Encode / Decode
  • In-Memory KV
  • Duplicate Files
  • KV Store
  • Monster Team Battle
  • GPU Credit Tracker v1
  • In-Memory DB · SQL
  • Auth Mgr · Sessions
  • KV Store · Thread-Safe
  • KV Store · Persistent
  • LRU Cache
  • Stack Samples
  • Sampling Profiler
  • Duplicate Files+
  • KV Store · Chunked
  • Dasher Payout
TTL / Expiry
  • Time-Based KV
  • Auth Mgr · TTL
  • Meeting Rooms
  • GPU Credit Tracker v2
Dependency Graph
  • Unix cd
  • Spreadsheet
  • Follow Graph
Resumable Iterator
  • IPv4 Iterator
  • Resumable Iterator
Grid / Graph
  • Disease Spread
  • N-ary Clusters
  • LCA Family
  • Vertical Order
  • Nested List Weights
  • Web Crawler
Other patterns
  • Semantic Version
  • Image Pipeline
  • Image Pipeline 2
  • Basic Calculator
  • Merge Intervals
  • Palindrome Deletions
  • Weighted Random
  • Kth Largest
  • Valid Parens
  • The Android App Template
  • Weather App
  • Notes App
  • Feature Flag SDK
  • Analytics / Logging SDK
  • Crash Reporter SDK
  • HTTP Client Library
  • Image Loading Library
  • Messenger App
  • Photo Gallery App
  • Music Player App
  • Doc Editor App
  • Newsfeed App
  • Ride-Sharing App
  • Autonomous Ride-Sharing App
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New content

10 new coding challenges

Ten new multi-level coding challenges - Basic Calculator, LRU Cache, Merge Intervals Family, Lowest Common Ancestor Family, Binary Tree Vertical Order, Valid Palindrome With Deletions, Weighted Random Pick, Nested List Weights, Kth Largest Element, and Dasher Payout Engine.

Released1mo ago
New content

17 new DSA practice questions

Seventeen new LeetCode-style practice questions across the DSA core concepts - arrays, binary search, bit manipulation, DFS, dynamic programming, graphs, greedy, hash maps, queues, recursion, sorting, trees, and union find - each with labeled examples, constraints, and a Solution Breakdown.

Released1mo ago
Update

DSA pages aligned - canonical examples and Solution Breakdown

Every DSA core concept page now follows one structure - standalone canonical example pages, LeetCode-style problem formatting with labeled examples and constraints, a renamed Solution Breakdown section, and difficulty-ordered question sets.

Released1mo ago
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Full breakdowns, AI-guided practice on a live canvas, and mock interviews for Android system design and coding rounds.

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53 questions
Companies
  • Stack Samples → Trace Events

    A sampling profiler captures call stacks at fixed intervals. Convert those snapshots into start/end span events. Three phases - basic diff, recursion, then a min-consecutive-count debounce.

    Coding
    hard
    Anthropic
    5mo
  • In-Memory Key-Value Store

    Build a key-value store as a single class. Layer on prefix-scan, TTL, then gzip persistence - each phase is its own surface area on top of the previous one.

    Coding
    medium
    OpenAI
    5mo
  • Sampling Profiler → Trace Events (Sample/Event types)

    A typed variant of the stack-samples problem - Sample and Event are dataclasses, comparisons are by value, and the contract is exposed via the imports the test file requires.

    Coding
    hard
    Anthropic
    5mo
  • Find Duplicate Files

    Group identical files by content, keeping memory bounded while scanning, then add size pre-filtering and an opt-in symlink/min-size policy.

    Coding
    medium
    AnthropicGoogle
    5mo
  • Find Duplicate Files - Extended

    Same starting point as the basic duplicate-finder, then six layers: bounded-memory hashing, size + symlink filters, recursive directory walking, full-content confirmation, per-file error handling, and a persistent hash cache.

    Coding
    hard
    Anthropic
    5mo
  • Encode and Decode Strings

    Serialize a list of arbitrary strings into a single string and parse it back. Any character is allowed in the strings, including the delimiters - the format must round-trip them all.

    Coding
    easy
    GoogleBloomberg
    5mo
  • KV Store - Serialize and Deserialize

    An in-memory key-value store that checkpoints its full state to a single string and restores from it. Both keys and values are arbitrary strings - delimiters, newlines, unicode, anything. The on-the-wire format must round-trip them all.

    Coding
    medium
    OpenAI
    5mo
  • Monster Team Battle

    Auto-simulate a turn-based team fight enforcing round order, dying-strike counterattacks, draw detection, and a full time-ordered event log.

    Coding
    medium
    OpenAI
    5mo
  • GPU Credit Tracker: Variant 1

    Historical-balance lookup (add/deduct stamped at a time, query balance as of t) and FIFO consumption with per-batch expiration - two classes covering the two sides of a credit system.

    Coding
    medium
    OpenAI
    5mo
  • In-Memory Database with SQL-like Operations

    A single-table in-memory database supporting INSERT and SELECT with WHERE (AND-only equality) and multi-column ORDER BY, with the query results guaranteed deterministic.

    Coding
    medium
    AnthropicOpenAI
    5mo
  • Authentication Manager - Sessions

    Session-based auth: register, login, logout, session TTL, and salted password hashing. Often paired with the TTL token variant.

    Coding
    medium
    OpenAI
    5mo
  • KV Store - Chunked Persistence

    The chunked-persistence follow-up to KV Store Serialize/Deserialize. Serialize a key-value store with a fixed-width binary (TLV) format, then split the blob into 1 KB chunks and checkpoint it to an S3-style object store - with the chunk count carried in a chunk header, no separate metadata object.

    Coding
    hard
    OpenAI
    4mo
  • KV Store - Thread-Safe

    The concurrency follow-up to the in-memory key-value store. Many threads call put / get / erase on one shared store at once; make it correct under contention, then refine from a single global lock to a reader-writer lock and finally to atomic compound operations (get_and_set, compare_and_swap).

    Coding
    medium
    OpenAINetflix
    4mo
  • KV Store - Persistent

    The durability follow-up to the in-memory key-value store. Snapshot the store to disk and restore it, then make the write crash-safe with an atomic temp-file rename, then add an append-only write-ahead log so writes made since the last snapshot survive a crash. Arbitrary key/value bytes must round-trip across disk intact.

    Coding
    medium
    OpenAI
    4mo
  • LRU Cache

    Bounded-memory caching: get it correct, then evict the least recently used, then make every operation O(1), then serve recency queries and TTL expiry.

    Coding
    medium
    MetaAmazon
    1mo
  • Dasher Payout Engine

    Courier pay from raw delivery events: pair events into active intervals, pay per active minute, pay a bonus for minutes juggling concurrent deliveries, then survive out-of-order, duplicate, and never-ended events, and finally emit an auditable breakdown.

    Coding
    hard
    DoorDash
    1mo
  • Time-Based Key-Value Store

    A key-value store with timestamped writes. Read at time t returns the latest value written at or before t; later phases add arbitrary-order inserts and range queries.

    Coding
    medium
    OpenAIGoogle
    5mo
  • Authentication Manager - TTL

    Tokens issued with a fixed TTL; renew before expiry to extend; count live tokens at time t. Later phases make the count efficient under many issued tokens and add thread safety.

    Coding
    medium
    OpenAIX
    5mo
  • Meeting Rooms

    Given a list of half-open meeting intervals, find the minimum number of conference rooms; later phases assign specific room IDs and add capped capacity with priority eviction.

    Coding
    medium
    OpenAIGoogle
    5mo
  • GPU Credit Tracker: Variant 2

    Single-account GPU credit grants with time windows; FIFO partial spending and windowed balance queries.

    Coding
    medium
    OpenAI
    4mo
  • Spreadsheet with Formulas

    Cells hold literals or formulas that reference other cells. set_cell must evaluate each cell so get_cell is fast; later levels propagate changes to dependents and reject updates that would form a cycle.

    Coding
    hard
    OpenAIAirbnb
    5mo
  • Unix cd with Symlink Resolution

    A stateful shell that supports POSIX cd semantics - absolute and relative paths, `.`/`..`, symlink resolution, and cycle detection.

    Coding
    medium
    OpenAIMetaAnthropic
    5mo
  • Follow Graph with Snapshots

    A mutable social graph that has to answer point-in-time history without storing a full copy per snapshot, and rank friends-of-friends recommendations both live and as-of a past snapshot.

    Coding
    hard
    OpenAI
    4mo
  • IPv4 Address Iterator and CIDR

    Convert dotted-quad to 32-bit integers and back, iterate forward and reverse over a range, parse CIDR notation, and decompose an arbitrary range into the minimum CIDR cover.

    Coding
    medium
    OpenAIAirbnbAnthropic
    5mo
  • Resumable Iterator

    An iterator whose state can be captured and replayed - stop iterating, save the state, hand it to a fresh iterator, and continue from exactly where you left off. Layer composition over flat and nested data.

    Coding
    medium
    OpenAI
    5mo
  • Disease Spread in a Grid

    A cell becomes infected on step t+1 only if it has at least K infected neighbors right now. Simulate the outbreak, detect when it stalls, and survive boundary and edge shapes.

    Coding
    medium
    OpenAI
    5mo
  • Async N-ary Cluster Count

    Count nodes in an n-ary tree using async fan-out: child counts must run concurrently so wall-clock time scales with tree depth, not node count.

    Coding
    medium
    OpenAI
    5mo
  • Web Crawler

    Build a web crawler on top of an injected get_links helper - breadth-first crawl with deduplication, then same-domain and fragment handling, then a concurrent version with a thread pool and a shared visited set, and finally depth and global rate limits.

    Coding
    hard
    AnthropicOpenAI
    4mo
  • Lowest Common Ancestor Family

    Five levels of lowest-common-ancestor questions: on a binary search tree, on general binary trees, from parent pointers alone, across n-ary org charts, and in bulk batches against a static tree.

    Coding
    medium
    MetaAmazon
    1mo
  • Binary Tree Vertical Order

    Group a binary tree's values into vertical columns: basic column grouping first, then a deterministic top-to-bottom row order, then value tie-breaks, then tree shapes deep enough to strain naive traversals.

    Coding
    medium
    MetaBloombergUber
    1mo
  • Nested List Weights

    A nested list of integers gets three contracts - depth-weighted sum, reverse-weighted sum, then a flatten iterator that yields one integer at a time - with nesting deep enough to strain naive traversals.

    Coding
    medium
    MetaLinkedIn
    1mo
  • Valid Parentheses

    Five escalating takes on one bracket string: validate it, point editor squigglies at it, mine the longest valid substring, repair a broken file, and validate with a corruptable wildcard.

    Coding
    hard
    GoogleBloomberg
    5mo
  • Semantic Version Comparison and Constraints

    Parse and compare semver-style version strings, then layer on range constraints (>=, <, ^, ~, x-wildcards).

    Coding
    medium
    5mo
  • Image Processing Pipeline

    Apply ordered image operations (resize, rotate, crop, blur, grayscale) with Pillow - one image, then every image-by-pipeline combination, then a thread pool, then partial-failure isolation, then directory discovery.

    Coding
    medium
    Anthropic
    4mo
  • Image Pipeline - Recursive Batch

    The directory-scale follow-up to Image Processing Pipeline. Walk a tree of images (Large/ and Small/ subfolders), load transform lists from a folder of JSON files, and apply every transform to every image in parallel - preserving the source subdirectory layout in the output and isolating per-combination failures, at a scale of millions of combinations.

    Coding
    medium
    Anthropic
    4mo
  • Basic Calculator

    Evaluate arithmetic expressions as raw text: addition and subtraction first, then multiplication and division, then parentheses, then unary signs and malformed input.

    Coding
    medium
    MetaGoogleAmazon
    1mo
  • Merge Intervals Family

    Build a clean merged view of overlapping calendar intervals, then answer the queries production systems ask of it: insert a new interval, intersect two schedules, and find when everyone is free - each level a new access pattern on the same half-open interval model.

    Coding
    medium
    GoogleMeta
    1mo
  • Valid Palindrome With Deletions

    One string, four contracts: is it a palindrome, can it become one by deleting at most one character, at most k characters, and what is the exact minimum number of deletions it needs?

    Coding
    medium
    MetaMicrosoft
    1mo
  • Weighted Random Pick

    Pick one option at random, with each option's chance proportional to its weight - uniform baseline first, then weighted picks, then a picker that tolerates live weight updates, adversarial edge cases, and a 2-D area-sampling generalization.

    Coding
    medium
    MetaAmazon
    1mo
  • Kth Largest Element

    Return the kth largest element from a stream of scores, where k=1 is the maximum and duplicates count separately. Levels tighten the budget - bounded memory, average-linear time, and answers that stay current as scores keep arriving.

    Coding
    medium
    MetaLinkedInAmazon
    1mo
  • Weather App

    A forecast app that loads instantly from cache, shows an honest offline state, and refreshes in the background.

    Android System Design
    easy
    HubSpotAmazon
    4mo
  • Notes App

    A notes app that saves as you type. Local-first, synced to the cloud when the network is available.

    Android System Design
    easy
    Netflix
    4mo
  • Feature Flag SDK

    Toggle features from the server without shipping a new build. Typed, deterministic, and never blocks the UI.

    Android System Design
    medium
    AmazonMicrosoftStripe
    5mo
  • Analytics / Logging SDK

    A tracking SDK that never drops an event, never blocks the main thread, and plays nice with battery and consent.

    Android System Design
    hard
    GoogleMetaMicrosoft
    5mo
  • Crash Reporter SDK

    Capture crashes no one is around to see. Stack trace, device state, and a durable queue that survives the kill.

    Android System Design
    hard
    5mo
  • HTTP Client Library

    The shared layer every network call flows through. Reuse connections, retry safely, never hang forever.

    Android System Design
    hard
    Meta
    5mo
  • Image Loading Library

    Load and cache images without jank or OOM. The library every app needs but few think to design.

    Android System Design
    hard
    Meta
    5mo
  • Messenger App

    Send a message, trust it arrives. An outbox that survives process kills and ordering that holds across reconnects.

    Android System Design
    hard
    GoogleMeta
    5mo
  • Photo Gallery App

    A gallery over device and cloud photos. One timeline, background backup, instant full-resolution viewing.

    Android System Design
    hard
    GoogleMeta
    4mo
  • Music Player App

    Stream and play music on Android. Gapless, background-capable, offline-ready, and respectful of audio focus.

    Android System Design
    hard
    AmazonSpotifyDatabricks
    5mo
  • Doc Editor App

    A collaborative text editor on Android. Type locally at 60 fps, merge conflicts across hours of offline.

    Android System Design
    hard
    GoogleAmazonNotion
    5mo
  • Newsfeed App

    An infinite social feed that scrolls at 60 fps, paginates without losing your place, and handles optimistic likes.

    Android System Design
    hard
    GoogleBloomberg
    5mo
  • Ride-Sharing App

    A rider app where the server owns the ride state. Request survives a kill, driver positions stream smoothly.

    Android System Design
    hard
    GoogleMeta
    5mo