Tree Traversal Interview Patterns: Inorder, Level Order, Zigzag, and Serialize/Deserialize (2025)
The Four Traversals Binary tree traversals: Inorder (left, root, right) — gives sorted order for a BST. Preorder (root, left, […]
Algorithms
System Design
System Design: Coupon and Promo Code System — Validation, Redemption, and Abuse Prevention
Core Requirements A coupon system allows businesses to issue discount codes that customers apply at checkout. Types: percentage discount (20%
System Design
Low-Level Design: Bank Account System — Transactions, Overdraft Protection, and Interest Calculation
Core Entities Customer: customer_id, name, date_of_birth, ssn_hash, address, kyc_status (PENDING, VERIFIED, REJECTED). Account: account_id, customer_id, account_type (CHECKING, SAVINGS, MONEY_MARKET), status
Algorithms
Dynamic Programming Patterns: Recognizing and Solving DP Problems (Complete Guide 2025)
Recognizing DP Problems Dynamic programming applies when a problem has two properties: (1) Optimal substructure: the optimal solution to the
System Design
System Design: Audit Log — Immutable Event Trail, Compliance, and Tamper Detection
Why Audit Logs? An audit log records every significant action in a system: who did what, to which resource, when,
System Design
Low-Level Design: Ride-Sharing App (Uber/Lyft) — Driver Matching, Surge Pricing, and Trip State Machine
Core Entities Rider: rider_id, name, phone, email, payment_method_id, rating, total_trips. Driver: driver_id, name, phone, license_number, vehicle_id, status (OFFLINE, AVAILABLE, ON_TRIP),
Algorithms
Graph Topological Sort Interview Patterns: Kahn’s Algorithm, DFS Post-Order, and Cycle Detection (2025)
What is Topological Sort? A topological sort of a Directed Acyclic Graph (DAG) is a linear ordering of vertices such
System Design
System Design: Flash Sale — High-Concurrency Inventory, Queue-Based Purchase, and Oversell Prevention
The Flash Sale Problem A flash sale offers a limited quantity of items at a steep discount for a short
System Design
Low-Level Design: Task Management System (Trello/Jira) — Boards, Workflows, and Notifications
Core Entities Workspace: workspace_id, name, owner_id, plan (FREE, PRO, ENTERPRISE), created_at. Board: board_id, workspace_id, name, description, visibility (PUBLIC, PRIVATE, WORKSPACE),
Algorithms
String Hashing Interview Patterns: Rabin-Karp, Rolling Hash, and Polynomial Hashing (2025)
Why String Hashing? Naive string matching (check every position in the text for the pattern) is O(n*m) where n =
System Design
System Design: Inventory Management System — Stock Tracking, Reservations, and Reorder Automation
Core Requirements An inventory management system tracks the quantity of products across one or more warehouses, coordinates stock reservations during
System Design
Low-Level Design: Online Auction System (eBay) — Bidding, Reserve Price, and Sniping Prevention
Core Entities Auction: auction_id, seller_id, item_id, title, description, start_price, reserve_price (nullable, minimum price to sell), buy_now_price (nullable, instant purchase), start_time,
Algorithms
Graph BFS Interview Patterns: Shortest Path, Islands, Word Ladder, and Multi-Source BFS (2025)
BFS Fundamentals BFS (Breadth-First Search) explores a graph level by level, visiting all nodes at distance 1 before distance 2,
System Design
System Design: Digital Wallet Service (Venmo/CashApp) — Transfers, Ledger, and Consistency
Core Requirements A digital wallet holds a user’s balance and enables peer-to-peer transfers. Key operations: add money (top-up from a
System Design
Low-Level Design: Online Learning Platform (Coursera/Udemy) — Courses, Progress, and Certificates
Core Entities Course: course_id, title, description, instructor_id, category, level (BEGINNER, INTERMEDIATE, ADVANCED), language, price, published_at, status (DRAFT, PUBLISHED, ARCHIVED), total_enrolled.