Two Sum (Hashmap)

Question

## Problem

Return indices of two numbers summing to a target; explain the STL container you'd use in C++.

## Example
```
[2,7,11,15], 9 -> [0,1]
```

## Constraints
- 2 ≤ n ≤ 10^4

## Note
Zoom probes C++/STL knowledge (e.g., 'what is the STL?'). Use unordered_map and justify it.