9 min read Core Entities Two primary entities drive the design: Token Bucket Algorithm Tokens refill at rate r tokens/second up to capacity […] Read article
6 min read Chinese Remainder Theorem (CRT) CRT solves systems of congruences: given x ≡ ai (mod mi) where mi are pairwise coprime, Read article
6 min read Core Entities Driver: driver_id, name, vehicle_type (ECONOMY/COMFORT/XL), status (OFFLINE, AVAILABLE, ON_TRIP), current_location (POINT: lat/lng), last_location_update, rating, acceptance_rate. Rider: rider_id, name, Read article
5 min read Core Entities and Fan-Out Strategies Post: post_id, author_id, content, media_urls (array), created_at, like_count, comment_count, share_count. Follow: follower_id, followee_id, created_at. FeedItem: Read article
5 min read Requirements and Scale Functional: turn-by-turn navigation, ETA estimation, real-time traffic updates, points of interest search, route alternatives. Non-functional: 1B daily Read article
6 min read Backtracking Template Backtracking = DFS + undo. The universal template: choose (add to path), recurse (explore), unchoose (remove from path). Read article
5 min read Core Challenge and Entities A flash sale creates a sudden traffic spike (100x-1000x normal load) competing for limited inventory. The Read article
6 min read Requirements and Core Challenge Functional: multiple users edit the same document simultaneously, changes from all users are reflected in real Read article
8 min read Why Sorting Algorithms Still Matter in Interviews Interviewers at top tech companies still ask about sorting not to test memorization, Read article
8 min read Scope and Requirements A payment processing platform handles money movement between buyers and sellers through external payment processors (Stripe, Adyen, Read article
10 min read Problem Statement Design a content moderation system for a social platform. User-generated content (text, images, video) must be screened before Read article
6 min read Cycle Detection in Undirected Graphs from collections import deque # DFS approach: track parent to distinguish back edge from tree Read article
5 min read 2D Grid BFS and DFS Fundamentals from collections import deque DIRS = [(0,1),(0,-1),(1,0),(-1,0)] # right, left, down, up def bfs_grid(grid, Read article
6 min read Core Entities Provider: provider_id, name, email, type (DOCTOR, THERAPIST, CONSULTANT, TUTOR), timezone, settings (JSONB: buffer_minutes_between_appointments, max_advance_days, min_notice_hours, auto_confirm). Schedule: schedule_id, Read article
6 min read Requirements and Scale Functional: upload raw video, transcode to multiple resolutions (240p/480p/720p/1080p/4K), store and serve video streams, support adaptive bitrate Read article
