Topological Sort and Strongly Connected Components: Interview Patterns for Directed Graphs (2025)
Topological Sort A topological ordering of a directed acyclic graph (DAG) is a linear ordering of vertices such that for […]
Algorithms
System Design
System Design: Distributed Transactions — Two-Phase Commit, Saga Pattern, and the Outbox Pattern
The Problem with Distributed Transactions In a monolith, a database transaction is atomic: either all changes commit or all roll
System Design
Low-Level Design: Analytics Dashboard — Metrics Aggregation, Time-Series Storage, and Real-Time Charting
Core Entities Metric: metric_id, name, type (COUNTER, GAUGE, HISTOGRAM), unit, description, owner_id, created_at. DataPoint: metric_id, timestamp, value (DOUBLE), tags (key-value
Algorithms
Number Theory Interview Patterns: GCD, Sieve of Eratosthenes, Fast Exponentiation, and Modular Arithmetic (2025)
When Number Theory Appears in Interviews Number theory problems appear in interviews at companies like Google, Facebook, and competitive programming
System Design
Low-Level Design: Survey and Form Builder — Dynamic Forms, Response Collection, and Analytics
Core Entities Survey: survey_id, owner_id, title, description, status (DRAFT, ACTIVE, CLOSED), settings (allow_multiple_responses, anonymous, show_progress_bar), created_at, closes_at. Question: question_id, survey_id,
System Design
Low-Level Design: Content Management System — Drafts, Versioning, Roles, and Publishing Workflow
Core Entities ContentType: type_id, name (Article, Page, Product), fields_schema (JSON schema defining the fields). Content: content_id, type_id, slug, status (DRAFT,
Algorithms
Minimum Spanning Tree Interview Patterns: Kruskal, Prim, and Network Design Problems (2025)
What Is a Minimum Spanning Tree? A Minimum Spanning Tree (MST) of a weighted undirected graph is a spanning tree
System Design
Low-Level Design: Social Media Platform — Posts, Feeds, Follows, and Notifications
Core Entities User: user_id, username, display_name, bio, avatar_url, follower_count, following_count, created_at. Post: post_id, author_id, content, media_urls[], like_count, comment_count, share_count, visibility
Algorithms
Shortest Path Interview Patterns: Dijkstra, Bellman-Ford, A*, and Floyd-Warshall (2025)
Choosing the Right Algorithm Decision tree: non-negative weights? -> Dijkstra (O((V+E) log V)). Negative weights, no negative cycles? -> Bellman-Ford
System Design
Low-Level Design: Stock Trading Platform — Order Book, Matching Engine, and Portfolio Management
Core Entities Order: order_id, user_id, symbol, side (BUY, SELL), order_type (MARKET, LIMIT, STOP_LIMIT, TRAILING_STOP), quantity, limit_price, stop_price, status (PENDING, PARTIALLY_FILLED,
Algorithms
Monotonic Stack and Queue Interview Patterns: Next Greater Element, Largest Rectangle, Sliding Window Maximum (2025)
Monotonic Stack Concept A monotonic stack maintains elements in strictly increasing or decreasing order. Elements that violate the order are
System Design
System Design: ML Feature Store — Feature Computation, Storage, Serving, and Point-in-Time Correctness
What Is a Feature Store? A feature store is a centralized platform for computing, storing, serving, and sharing ML features.
System Design
Low-Level Design: Subscription Box Service — Curation, Billing Cycles, Inventory Allocation, and Churn
Core Entities Subscriber: subscriber_id, user_id, plan_id, status (ACTIVE, PAUSED, CANCELLED), billing_cycle_anchor, next_billing_date, shipping_address, preferences[], skip_count. Plan: plan_id, name, price_cents, billing_interval
System Design
Low-Level Design: Food Delivery Platform — Orders, Driver Dispatch, Real-Time Tracking
Core Entities Restaurant: restaurant_id, name, address, geo_location, cuisine_type, is_open, prep_time_minutes, delivery_radius_km. MenuItem: item_id, restaurant_id, name, price_cents, category, is_available, customizations[]. Order:
System Design
Low-Level Design: Healthcare Appointment Booking — Scheduling, Reminders, EMR Integration
Core Entities Provider: provider_id, name, specialty, clinic_id, npi_number, languages[], accepts_new_patients. ProviderSchedule: schedule_id, provider_id, day_of_week, start_time, end_time, slot_duration_minutes, is_active. Appointment: appointment_id,