Team Challenge: Solve Two Sum (LeetCode #1)

## Problem

**Two Sum** (LeetCode #1)
🔗 https://leetcode.com/problems/two-sum/

Given an array of integers `nums` and an integer `target`, return the indices of the two numbers that add up to `target`. You can assume exactly one valid answer exists, and you can't use the same element twice.

The full problem statement is posted at `DSA/problems/1.py`.

## Task

This one's assigned to everyone. Read the problem, solve it, and submit your own solution.

- Problem file (read only, don't edit): `DSA/problems/1.py`
- Your solution goes in: `DSA//1.py`
- Language: Python or C++, your pick (if you go with C++, name your file `1.cpp` instead, since a `.py` file can't hold C++ code)

## Concepts to Brush Up On

- Arrays: how they're indexed and traversed
- Hash Maps / Hash Tables: the key to turning the brute force O(n²) approach into an O(n) one
- Time and space complexity (Big-O)
- OOP basics in Python or C++: LeetCode wraps every solution inside a `class Solution`, so knowing how classes and methods work matters here

## Lectures & Resources

**Problem walkthrough**
- NeetCode, Two Sum solution and explanation (video): https://www.youtube.com/watch?v=KLlXCFG5TnA
- NeetCode, written solution covering brute force, sorting, and hash map approaches: https://neetcode.io/solutions/two-sum

**Hash Tables**
- Abdul Bari, Hashing Technique Simplified: https://www.youtube.com/watch?v=mFY0J5W8Udk

**Arrays & DSA Fundamentals**
- Abdul Bari, full Data Structures & Algorithms playlist: https://www.youtube.com/playlist?list=PLsr8vTgyLdy_YndxNcI4WkH5Vorj5qvrv

**OOP in Python**
- Corey Schafer, Classes and Instances: https://www.youtube.com/watch?v=ZDa-Z5JzLYM
- Full Python OOP playlist: https://www.youtube.com/playlist?list=PL-osiE80TeTsqhIuOqKhwlXsIBIdSeYtc

**OOP in C++**
- freeCodeCamp (taught by CodeBeauty), Object Oriented Programming in C++: https://www.youtube.com/watch?v=wN0x9eZLix4

Feel free to study from any resources that you may find useful. Also give **[Apna College](https://www.youtube.com/@ApnaCollegeOfficial/playlists?themeRefresh=1)** a shot

## How to Submit

1. Pull the latest `main`
2. Create a branch: `git checkout -b dsa/1-two-sum-`
3. Create your folder if you don't have one yet: `DSA//`
4. Add your solution as `DSA//1.py` (or `1.cpp`)
5. Test it locally against the examples in `DSA/problems/1.py`
6. Commit: `git commit -m "Add solution for 1-two-sum"`
7. Push and open a PR against `main`. Link this issue in the PR description (e.g. `Refs #`)

## Checklist

- [ ] Read `DSA/problems/1.py`
- [ ] Reviewed at least the hash map and OOP resources above
- [ ] Solution added at `DSA//1.py`
- [ ] Tested locally
- [ ] PR opened and linked to this issue

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Everyone in the group is expected to attempt this one. If you get stuck, comment here instead of going quiet, someone will jump in to help.