First Duplicate in O(n) Time, O(1) Space

Question

## Problem

Given an array where values are in [1, n], find the first value that appears twice using O(1) extra space.

## Example
```
[2,3,3,1,5,2] -> 3
```

## Constraints
- Values in range [1, n]

## Approach
Index-as-hash: negate `nums[abs(v)-1]`; first already-negative hit is the duplicate.