[SUBMISSION][BACKEND]: Problem 4, 5, 6

src/problem4/index.js

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+/**
+ * Provide 3 unique implementations of the following function in JavaScript.
+ *
+ * **Input**: `n` - any integer
+ *
+ * *Assuming this input will always produce a result lesser than `Number.MAX_SAFE_INTEGER`*.
+ *
+ * **Output**: `return` - summation to `n`, i.e. `sum_to_n(5) === 1 + 2 + 3 + 4 + 5 === 15`.
+ */
+
+/**
+ * Implementation 1: Iterative approach
+ * 
+ * Time Complexity: O(n)
+ * Space Complexity: O(1)
+ */
+var sum_to_n_a = function(n) {
+  let sum = 0;
+  for (let i = 1; i <= n; i++) {
+    sum += i;
+  }
+  return sum;
+};
+
+/**
+ * Implementation 2: Recursive approach
+ * 
+ * Time Complexity: O(n)
+ * Space Complexity: O(n)
+ */
+var sum_to_n_b = function(n) {
+  if (n === 1) {
+    return 1;
+  }
+  return n + sum_to_n_b(n - 1);
+};
+
+/**
+ * Implementation 3: Formula approach: 1 + 2 + ... + n = n * (n + 1) / 2
+ * 
+ * Time Complexity: O(1)
+ * Space Complexity: O(1)
+ */
+var sum_to_n_c = function(n) {
+  return (n * (n + 1)) / 2;
+};
+
+var a = sum_to_n_a(5); // 15
+var b = sum_to_n_b(5); // 15
+var c = sum_to_n_c(5); // 15
+
+console.log(a, b, c); // 15 15 15

src/problem4/readme.md

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+Command run
+```code
+node index
+```

src/problem5/.dockerignore

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+node_modules
+dist
+.env
+*.md
+.git
+.gitignore
+docker-compose.yml

src/problem5/.eslintrc.js

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+module.exports = {
+  parser: '@typescript-eslint/parser',
+  parserOptions: {
+    project: 'tsconfig.json',
+    sourceType: 'module',
+  },
+  plugins: ['@typescript-eslint/eslint-plugin'],
+  extends: [
+    'plugin:@typescript-eslint/recommended',
+    'plugin:prettier/recommended',
+  ],
+  root: true,
+  env: {
+    node: true,
+    jest: true,
+  },
+  ignorePatterns: ['.eslintrc.js'],
+  rules: {
+    '@typescript-eslint/linebreak-style': 0,
+    '@typescript-eslint/semi': 'off',
+    '@typescript-eslint/interface-name-prefix': 'off',
+    '@typescript-eslint/explicit-function-return-type': 'off',
+    '@typescript-eslint/explicit-module-boundary-types': 'off',
+    '@typescript-eslint/no-explicit-any': 'off',
+    'no-unused-vars': 'off',
+    '@typescript-eslint/no-unused-vars': [
+      'warn',
+      {
+        argsIgnorePattern: '^_',
+        varsIgnorePattern: '^_',
+        caughtErrorsIgnorePattern: '^_',
+      },
+    ],
+  },
+}

src/problem5/.gitignore

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+node_modules
+dist
+.env

src/problem5/.prettierrc

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+{
+  "singleQuote": true,
+  "semi": false,
+  "endOfLine": "auto",
+  "trailingComma": "all"
+}

src/problem5/Dockerfile

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+# ---- build stage ----
+FROM node:22-alpine AS builder
+
+WORKDIR /app
+
+COPY package*.json ./
+RUN npm ci
+
+COPY tsconfig.json ./
+COPY src ./src
+RUN npm run build
+
+# ---- development stage ----
+FROM node:22-alpine AS development
+
+ENV NODE_ENV=development
+
+WORKDIR /app
+
+COPY package*.json ./
+RUN npm ci
+
+COPY --from=builder /app/dist ./dist
+
+EXPOSE 3000
+
+CMD ["node", "dist/index.js"]
+
+
+# ---- production stage ----
+FROM node:22-alpine AS production
+
+ENV NODE_ENV=production
+
+WORKDIR /app
+
+COPY package*.json ./
+RUN npm ci --omit=dev
+
+COPY --from=builder /app/dist ./dist
+
+EXPOSE 3000
+
+CMD ["node", "dist/index.js"]

src/problem5/README.md

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+# Resource CRUD API
+
+REST API built with Express, TypeScript, TypeORM, and PostgreSQL.
+
+## Stack
+
+- **Runtime**: Node.js 22
+- **Framework**: Express 5
+- **ORM**: TypeORM 0.3
+- **Database**: PostgreSQL 16
+- **Language**: TypeScript 5
+
+## Project Structure
+
+```
+src/
+├── controllers/     # Route handlers, input validation
+├── services/        # Business logic
+├── repositories/    # Database queries
+├── entities/        # TypeORM entities
+├── dtos/            # Request/response data transfer objects
+├── routes/          # Express routers
+├── database/        # DataSource configuration and migrations
+├── utils/           # Snowflake ID generator, response helpers
+└── swagger.ts       # OpenAPI 3.0 spec
+```
+
+---
+
+## Setup
+
+### Prerequisites
+
+- Node.js 22+
+- PostgreSQL 16+ (or Docker)
+
+### Install dependencies
+
+```sh
+npm install
+```
+
+### Environment variables
+
+Create a `.env` file in the project root:
+
+```env
+NODE_ENV=development
+PORT=3000
+DB_HOST=localhost
+DB_PORT=5432
+DB_USER=postgres
+DB_PASSWORD=postgres
+DB_NAME=postgres
+```
+
+### Run locally
+
+```sh
+npm run build
+npm run migration:run
+npm start
+```
+
+Or in watch mode during development:
+
+```sh
+npm run dev
+```
+
+---
+
+## Docker
+
+### Run with Docker Compose
+
+```sh
+docker compose up --build
+```
+
+This starts two services:
+
+| Service    | Port   |
+|------------|--------|
+| `api`      | `3000` |
+| `postgres` | `5432` |
+
+The API waits for PostgreSQL to pass its healthcheck before starting.
+
+### Stop
+
+```sh
+docker compose down
+```
+
+To also remove the database volume:
+
+```sh
+docker compose down -v
+```
+
+---
+
+## API Documentation (Swagger)
+
+Interactive API docs are available via Swagger UI once the server is running.
+
+```
+http://localhost:3000/docs
+```
+
+You can explore all endpoints, view request/response schemas, and execute requests directly from the browser.
+
+---
+
+## API Endpoints
+
+Base URL: `http://localhost:3000`
+
+### Create a resource
+
+```
+POST /resources
+```
+
+**Body**
+
+```json
+{
+  "name": "my-resource",
+  "status": "active"
+}
+```
+
+`status` is optional, defaults to `active`.
+
+**Response** `201`
+
+```json
+{
+  "id": "7318309505701888001",
+  "name": "my-resource",
+  "status": "active",
+  "created_at": "2024-01-01T00:00:00.000Z",
+  "updated_at": "2024-01-01T00:00:00.000Z"
+}
+```
+
+---
+
+### List resources
+
+```
+GET /resources?page=1&limit=20&status=active
+```
+
+**Query parameters**
+
+| Param    | Type     | Default  | Description                      |
+|----------|----------|----------|----------------------------------|
+| `page`   | integer  | `1`      | Page number                      |
+| `limit`  | integer  | `20`     | Items per page (max `100`)       |
+| `status` | `active` \| `inactive` | —  | Filter by status    |
+
+**Response** `200`
+
+```json
+{
+  "data": [...],
+  "total": 42,
+  "page": 1,
+  "limit": 20
+}
+```
+
+---
+
+### Get a resource
+
+```
+GET /resources/:id
+```
+
+**Response** `200`
+
+```json
+{
+  "id": "7318309505701888001",
+  "name": "my-resource",
+  "status": "active",
+  "created_at": "2024-01-01T00:00:00.000Z",
+  "updated_at": "2024-01-01T00:00:00.000Z"
+}
+```
+
+---
+
+### Update a resource
+
+```
+PATCH /resources/:id
+```
+
+**Body** (all fields optional)
+
+```json
+{
+  "name": "updated-name",
+  "status": "inactive"
+}
+```
+
+**Response** `200` — updated resource object.
+
+---
+
+### Delete a resource
+
+```
+DELETE /resources/:id
+```
+
+**Response** `204 No Content`
+
+---
+
+## Error Responses
+
+| Status | Description                        |
+|--------|------------------------------------|
+| `400`  | Validation error                   |
+| `404`  | Resource not found                 |
+| `500`  | Internal server error              |
+
+```json
+{
+  "message": "Resource with id 123 not found"
+}
+```