[BE] Hung Pham

[tienhung278]

Problem 4: Three Ways to Sum to N

Description:
Provides three distinct implementations of a function that computes the summation from 1 to n:

• sum_to_n_a — Uses Gauss's formula n*(n+1)/2. O(1) time, O(1) space.
• sum_to_n_b — Iterative approach with a for-loop accumulator. O(n) time, O(1) space.
• sum_to_n_c — Recursive approach. O(n) time, O(n) space (call stack).

Each function includes inline comments documenting its time and space complexity trade-offs.

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Problem 5: Todo CRUD API (Express.js + TypeScript)

Description:
A fully functional Express.js API written in TypeScript implementing all five CRUD operations for a Todo resource:

• Create (POST /todos) — Validates input, generates UUID, sets timestamps.
• List (GET /todos) — Supports filtering by completed status, text search, and pagination (limit/offset).
• Get Details (GET /todos/:id) — Returns a single todo or 404.
• Update (PUT /todos/:id) — Partial update with automatic updatedAt timestamp.
• Delete (DELETE /todos/:id) — Removes a todo by ID.

Architecture follows a layered pattern (Routes → Controllers → Services → Repositories) with:

• Repository interface for swappable data storage (currently JSON file-based)
• Custom error classes + global error handler middleware
• Constructor-based dependency injection
• Centralized configuration

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Problem 6: Scoreboard Module Specification

Description:
A comprehensive technical specification document for a live-updating scoreboard system, intended as a blueprint for backend engineers. Includes:

• System architecture diagram — Full request flow from client through auth, rate limiting, controller, service, database, cache, and WebSocket broadcast.
• Execution flow diagrams — For score updates, scoreboard retrieval, and WebSocket connections.
• API endpoint specs — POST /scores/action, GET /scores/top, GET /scores/me with request/response schemas.
• Database schema — Users, scores, and action_logs tables with indexes.
• Security measures — JWT authentication, per-user rate limiting, idempotency keys, HMAC-signed action payloads.
• Technology stack recommendations — Node.js, Redis sorted sets, PostgreSQL, Socket.io.
• Improvement suggestions — Horizontal scaling, anomaly detection, action verification queues, batch processing, graceful degradation, and testing strategy.