Missing Number in an Unsorted Array

Question

## Problem

Given an array containing `n` distinct numbers in [0, n], find the one that is missing.

## Example
```
[3,0,1] -> 2
```

## Constraints
- 0 ≤ n ≤ 10^4

## Approach
Sum formula `n(n+1)/2 - sum`, or XOR trick. Discuss overflow safety.