"Interviews - LLD Frameworks"

LLD Problem Solving Framework (Phase-wise with Examples)

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Phase 1: Understand the Problem (Time: 5 minutes)

Goal: Get absolute clarity on the problem statement.

Questions to ask:

• What are the inputs/outputs?

• What real-world entities exist?

• What is out of scope?

Examples:

Problem	Clarifying Questions
Parking Lot	Multi-level? Handicapped spots? Reservations?
Elevator	How many elevators? Multiple floors? Prioritization logic?
Expression Evaluator	Supported operators? Data types? What happens if field missing in input?

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Phase 2: Identify Core Responsibilities (Time: 5–10 minutes)

Goal: Break the problem into distinct functional components.

Examples:

Problem	Core Responsibilities
Parking Lot	Spot allocation, availability tracking, entry/exit flow
Elevator	Request handling, scheduling, movement control
Expression Evaluator	Parsing condition rules, comparing fields, applying operators

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Phase 3: Define Key Entities & Interfaces (Time: 10 minutes)

Goal: Design key classes, enums, and interfaces that model the system.

Examples:

Problem	Key Classes & Interfaces
Parking Lot	ParkingLot, Spot, Vehicle, Ticket, SpotAllocator
Elevator	Elevator, Request, Scheduler, Direction, ElevatorState
Expression Evaluator	Condition, Operator, ExpressionEvaluator, OperatorRegistry

Design Tip: Follow OOP principles — Identify areas for abstraction (via interfaces), encapsulation, and extensibility.

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Phase 4: Strategy Pattern or Plug-in Logic (Time: 10–15 minutes)

Goal: Make the system flexible and extensible using patterns like Strategy, Factory, or Registry.

Examples:

Problem	Plug-in Logic
Parking Lot	Use SpotAllocator interface with implementations for Nearest, Reserved, etc.
Elevator	Use Scheduler interface with FCFS, Optimized, or PriorityBased strategies
Expression Evaluator	Use Operator interface and plug different comparisons via strategy pattern

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Phase 5: Build Orchestrator / Engine (Time: 10–15 minutes)

Goal: Assemble all components into a working system that can be tested and extended.

Examples:

Problem	Core Engine Logic
Parking Lot	Entry/exit controller calling allocator and updating map of spots
Elevator	Scheduler assigns elevators, moves elevator state machine
Expression Evaluator	Iterate conditions → get operator → compare values → return result

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Phase 6: Validate & Extend (Time: 5 minutes)

Goal: Handle edge cases, support future extensions, and demonstrate understanding.

Examples:

Problem	Additions & Validations
Parking Lot	Support for electric vehicles, pricing strategies
Elevator	Fire alarm interrupt, maintenance mode, weight handling
Expression Evaluator	Support nested expressions (AND/OR), add new data types

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Total Time: 45–50 minutes

Phase	Time	Deliverable
Understand	5 mins	Clarified scope and constraints
Define Roles	5–10	Core responsibilities
Model Entities	10	Classes, enums, interfaces
Plug-in Logic	10–15	Strategy/Factory/Registry
Engine Design	10–15	Integrate the system
Extend/Test	5	Edge cases + extensibility plan

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Summary Table Across All 3 Examples:

Phase	Parking Lot	Elevator	Expression Evaluator
Entities	Spot, Vehicle, Ticket	Elevator, Request, Scheduler	Condition, Operator, Evaluator
Core Functions	Assign/release spot, track capacity	Schedule requests, move elevators	Compare fields with rules
Patterns Used	Strategy (Allocator)	Strategy (Scheduler), FSM for movement	Strategy (Operator), Registry
Extensibility Points	New spot types, pricing	Scheduler strategies, smart movement	New operators, nested expressions
Common Mistakes	No spot tracking per level/floor	No prioritization logic	Hardcoded logic instead of extensible