Two Sum / Hashmap Medium

Question

## Problem

Return indices of two numbers summing to a target.

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

## Constraints
- 2 ≤ n ≤ 10^4

## Approach
Hash map, O(n). Dropbox OA (CodeSignal) is timed and can be brutal — speed + correctness.