Two Sum / Edge-Case-Heavy Medium

Question

## Problem

A LC easy-to-medium problem (e.g., Two Sum variants) where catching edge cases and testing is the real bar.

## Example
```
[3,3], target=6 -> [0,1]
```

## Constraints
- Duplicates, negatives, empty input

## What HubSpot looks for
Clarity, edge cases, and running your own tests matter more than the cleverest solution.