[BE-Code Challenge]: Resolve problem 4, 5, 6 - Le Tuan Khang

.gitignore

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+src/problem4/node_modules/
+src/problem5/node_modules/
+

readme.md

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-# 99Tech Code Challenge #1 #
+# 99Tech Code Challenge #1
 
 Note that if you fork this repository, your responses may be publicly linked to this repo.  
-Please submit your application along with the solutions attached or linked.   
+Please submit your application along with the solutions attached or linked.
 
 It is important that you minimally attempt the problems, even if you do not arrive at a working solution.
 
-## Submission ##
+## Submission
+
 You can either provide a link to an online repository, attach the solution in your application, or whichever method you prefer.
 We're cool as long as we can view your solution without any pain.
+
+# Code Challenge
+
+## Requirements
+
+- Node.js >= 18
+- npm
+
+## Running the Solutions
+
+Clone the repository:
+
+```bash
+git clone <your-repository-link>
+cd code-challenge
+```
+
+---
+
+## Problem 4: Three Ways to Sum to N
+
+This problem demonstrates different approaches to calculate the sum from 1 to N.
+
+```bash
+cd src/problem4
+npm install
+npm run dev
+```
+
+---
+
+## Problem 5: A Crude Server
+
+A simple backend server implementing CRUD APIs.
+
+```bash
+cd src/problem5
+npm install
+npm run dev
+```
+
+Server will start at `http://localhost:3000`
+
+---
+
+## Problem 6: Architecture
+
+This section contains system design diagrams.
+
+```bash
+cd src/problem6/docs
+```
+
+Open the following files:
+
+- `architecture.png` - High-level system architecture
+- `sequence.png` - Request flow / sequence diagram
+
+Additional notes are located at:
+
+```bash
+cd src/problem6
+```
+
+---
+
+## Notes
+
+- Each problem is independent and can be run separately.
+- There are no shared dependencies between problems.
+- Ensure the correct Node.js version before running.

src/problem4/README.md

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+# Sum to N
+
+## Assumptions
+
+### Assume A
+
+- Supports negative input
+- Example:
+
+  ```
+  sum_to_n(-5) = -5 + -4 + -3 + -2 + -1 = -15
+  ```
+
+---
+
+### Assume B
+
+- Only supports non-negative input
+- Throws error if `n < 0`
+
+---
+
+## Run
+
+Install dependencies:
+
+```
+npm install
+```
+
+Run program:
+
+```
+npm run start
+```
+
+---
+
+## Usage
+
+- Enter an integer when prompted
+- Enter `q` to quit
+
+Example:
+
+```
+Enter n: 5
+Enter n: -5
+Enter n: q
+```