Machine Coding Round
Low Level Design interview questions, solved properly
The LLD round is where you turn a vague problem into clean classes and working code in under an hour. These walkthroughs show the full object oriented design for each classic problem: the core classes and their responsibilities, how they relate, the design patterns that actually fit, the tricky edge cases, and a code skeleton you can build from. Every page comes with a class diagram so you can see the whole structure at a glance.
14 designs · each card shows its classes and patterns
Parking Lot9 classesStrategy · FacadeA parking lot program with spot types, a nearest-spot index for each gate, a ticket from entry to exit, and prices you can swap. The Java program on this page proves that cars racing for the last spot never share a spot.read the design Elevator System7 classesState · StrategyA multi-elevator controller where a dispatcher picks the best car for each hall request and every car runs a SCAN/LOOK scheduler over separate up and down queues driven by a state machine.read the design Vending Machine11 classesState · StrategyAn object-oriented Vending Machine design where the machine moves through Idle, HasMoney, Dispensing and OutOfStock states using the State pattern, with an inventory of product slots and a change-making module for coins and notes.read the design Movie Ticket Booking (BookMyShow)12 classesStrategy · StateObject-oriented design of a movie ticket booking system where the hard part is locking selected seats with a timeout so two users never book the same seat under concurrency.read the design LRU Cache5 classesSentinel nodes · DecoratorTo design an LRU cache, use a hash map and a doubly linked list together. The map finds any key's node at once, and the list keeps the nodes in order of use, most recent at the front. get moves a node to the front, and put adds one at the front and drops the node at the back when the cache is full. Both take O(1) time, which means the time does not grow with the number of items stored.read the design ATM7 classesState · Chain of ResponsibilityAn ATM that moves through clear states, keeps a card after 3 wrong PINs, plans which notes to pay out before it takes any money, debits each withdrawal only once, and gives the money back if the cash never leaves the machine.read the design Chess7 classesInheritance with one method to fill in · CommandA chess game in which each piece knows how it moves, the game throws away any move that leaves your own king in check, castling, en passant and promotion follow the FIDE rules, every move can be undone, and the move generator is proved right by counting moves against published numbers.read the design LFU Cache4 classesBucketing by count · LRU inside each bucketTo design an LFU cache in O(1), keep three things: a map from key to node, a map from each use count to a doubly linked list of the keys with that count, and minFreq, the lowest count in use. A get or put moves a key from the list for count f to the list for f + 1. When the cache is full, evict from the back of the minFreq list, which is the least recently used key among the least used.read the design Library Management System7 classesStrategy · ObserverA library system. A Book is the title, and a BookCopy is one item on the shelf. Members borrow copies within limits set by their type. A returned copy goes to the first member waiting for it. Late fines come from a rule the library can change.read the design Logging Framework7 classesSingleton · FactoryA small logging library like Logback or Log4j 2: loggers named by dots that inherit their level from a parent, appenders that send lines to the console or a file, formatters that shape each line, and an async writer (one that works in the background) with a bounded queue that either waits or drops and counts what it dropped.read the design Rate Limiter8 classesStrategy · FactoryA rate limiter that runs inside one program. Three methods sit behind one interface. Each user gets their own limiter on first use. Limits depend on the user's plan. And a test clock lets you check the timing exactly.read the design Snake and Ladder6 classesStrategy · One map for snakes and laddersSnake and ladder on any board size. Snakes and ladders live in one map from start square to end square. The dice and the finishing rule can be swapped. The board refuses bad layouts, and seeded simulations are checked against exact maths.read the design Splitwise10 classesStrategy · FactoryAn app for sharing costs. It records who paid and who owes. It splits in three ways without losing a paisa to rounding. It keeps one balance per person, and settles a group in few payments.read the design Tic-Tac-Toe6 classesStrategy · Keeping the rules in one placeTic-tac-toe on a board of any size. Players sit behind one interface, so a person or a bot can play. The board only accepts legal moves. The game owns turns and the result. And the winner check is fast: it never scans the whole board.read the design
One design is not enough for the machine coding round.
Get all 200 questions in the System Design and Distributed Systems pack
Each question has why it is asked, a full answer, what the interviewer listens for, the follow-ups, the common mistakes and a diagram (268 diagrams in all). A PDF you keep, junior to staff level.
200 questions268 diagramsjunior to staff
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