Remove Duplicates Preserving Order

Question

## Problem

Given an array, return the unique elements in their original order (no sorting).

## Example
```
[3,1,3,4,1,5] -> [3,1,4,5]
```

## Constraints
- 1 ≤ n ≤ 10^5

## Approach
Hash set to track seen while iterating. Reported Workday OA question.