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Compiling brilliance
Did you know? An LRU cache is a HashMap + a doubly-linked list → O(1) get & put.
Compiling brilliance
Did you know? An LRU cache is a HashMap + a doubly-linked list → O(1) get & put.
LLD
Class-level design, data structures, and concurrency done right.
Concurrency patterns explained: producer-consumer, thread pool, future, read-write lock, thread-safe singleton, and a semaphore-bounded pool for safe multithreaded design.
Design patterns explained for interviews and low-level design: the Gang of Four patterns — factory, builder, singleton, adapter, decorator, strategy, observer, state, template method, command.
Object-oriented programming explained for interviews: encapsulation, abstraction, inheritance, polymorphism, and composition over inheritance (is-a vs has-a) with a worked payment-system example.
The SOLID design principles explained with code: single responsibility, open/closed, Liskov substitution, interface segregation, dependency inversion, plus DRY, KISS and YAGNI for low-level design.
The Uber ride concurrency interview question: seat ride-request threads into cabs of four — 4+0, 0+4, or 2+2 only — with a mutex, two semaphores, and a barrier. Deadlock-free, stress-tested.
The two ideas at the core of a game engine: a fixed-timestep loop that makes simulation deterministic at any frame rate, and an Entity-Component-System that composes behaviour without inheritance.
A low-level design walkthrough of a web crawler core: the frontier queue, URL normalization before deduplication, a shared seen-set, per-host politeness, and termination by in-flight count.
A low-level design walkthrough of an online voting system: separating eligibility from the ballot so one vote per voter never links who voted to what, with an append-only, recountable tally.
A low-level design walkthrough of audio fingerprinting like Shazam: reducing a spectrogram to peak landmarks, hashing point pairs, and matching a noisy snippet by voting on a consistent offset.
A low-level design walkthrough of TOTP and Google Authenticator: a shared secret and the clock through HMAC to six digits, with an acceptance window that tolerates skew and blocks replay.
A low-level design walkthrough of a text editor core like Sublime: the gap buffer for fast cursor-local edits, plus undo/redo stacks built from edits that describe their own inverse.
A low-level design walkthrough of a download manager: splitting a file into byte-range segments, downloading them in parallel with retries, and resuming after a crash from an on-disk journal.
A low-level design walkthrough of a bar graph chart library: series data, a linear scale with nice ceilings, and pluggable renderers that draw the same chart in the console and as SVG.
A low-level design walkthrough of a coupon and promo system: eligibility rules as composable data, discounts with caps, the preview-redeem split, and usage limits that survive a rush.
A low-level design walkthrough of the Android unlock pattern: the crossing rule as a lookup table, pattern validation in O(1) per move, and counting all valid patterns with backtracking and symmetry.
A low-level design walkthrough of an online book reader system: a catalog of shared books, per-user sessions, page navigation that clamps at the covers, and the user-book bookmark as the hidden noun.
A low-level design walkthrough of a car rental system: reservations against category capacity instead of specific cars, interval counting on the calendar, pickup-time assignment, and late-fee billing.
A low-level design walkthrough of a thread pool: the bounded work queue, the worker loop that must never die, saturation and rejection, and the graceful shutdown promise — fully tested.
A low-level design walkthrough of a Splitwise expense sharing system: immutable expenses, derived balances, split strategies with the rounding paisa rule, and greedy debt simplification.
A low-level design walkthrough of a call center: rank-based dispatch across respondents, managers and a director, escalation that re-dispatches, queues per rank, and the freed agent who drains them.
A low-level design walkthrough of an elevator system: why first-come-first-served zigzags, the LOOK sweep algorithm, hall calls vs car calls, direction as state — fully implemented and tested.
A low-level design walkthrough of an AWS-style alarm and alert service: threshold rules as data, consecutive-breach debouncing, three honest states, and observers notified on transitions only.
A low-level design walkthrough of an airline booking system: identity inventory where seat 12A is nobody's substitute, the PNR as hidden noun, and seat holds that expire by the lazy clock.
A low-level design walkthrough of a hotel management system: half-open date ranges, the one-line overlap formula, booking a room type while the hotel assigns the room — fully implemented.
A low-level design walkthrough of an inventory management system: on-hand vs reserved vs available, two-phase reservations with commit and release, and the race that double-sells the last unit.
A low-level design walkthrough of a restaurant management system: seating by table fit, order tickets through a kitchen queue, and billing that frees the table — coordinated state machines.
A low-level design walkthrough of an in-memory file system: files and directories behind one Node interface, path resolution as a walk, recursive size via the Composite pattern — fully implemented.
A low-level design walkthrough of a JSON parser: the grammar as a call graph, recursive descent with a position cursor, escape and number handling, and error messages that point.
A low-level design walkthrough of a logging library: ordered levels, a cheap threshold gate, formatters, appenders as a Strategy, and the complete implementation of the log4j shape.
A low-level design walkthrough of an ATM: a state machine that retains cards after three wrong PINs, a bank service interface because the ATM owns no truth, and cash planned before any debit.
A low-level design walkthrough of chess: scoping the unbuildable, one movement rule per piece via the Strategy pattern, a shared path-clear helper for sliding pieces, and the complete implementation.
A low-level design walkthrough of Tetris: why the falling piece never lives on the board, rotation as pure arithmetic, line clears as a filter, and the complete implementation of the classic.
A low-level design walkthrough of a jackpot slot machine: independent reels with injected randomness, the paytable as data instead of if-chains, and a credits ledger that always balances.
A low-level design walkthrough of minesweeper: separating truth from view, the flood fill cascade done with discipline, first-click safety, and the complete implementation of the classic.
A low-level design walkthrough of the 2048 game: the merge-once rule everyone breaks, why all four directions are one mergeLeft function, win and lose detection, and the complete implementation.
A low-level design walkthrough of snake and ladder: why snakes and ladders are one Map, how injecting the die makes the game testable, and the complete implementation with a clean turn loop.
A low-level design walkthrough of a vending machine: coins, inventory, the can't-make-change trap, and when design patterns earn their keep — the honest path from if-checks to the State pattern.
A low-level design walkthrough of tic tac toe (Tic-Tac-Toe): an O(1) win check with counters, a clean state machine, and the complete implementation of the classic warm-up interview question.
How to approach any low-level design interview: turn requirements into classes, methods, and state machines — the parking lot classic, fully built — plus a cheat sheet for picking design patterns.
A low-level design walkthrough of a rate limiter: the token bucket class, the race condition that breaks it, making it thread-safe, a per-client registry, and the complete implementation.
A low-level design walkthrough of an O(1) LRU cache — a hash map plus a doubly linked list — how eviction works, the locking story, and the full browsable implementation.