Binary Search (and Variants)

Question

## Problem

Implement binary search; then find the first/last occurrence of a target in a sorted array with duplicates.

## Example
```
[1,2,2,2,3], target=2 -> first=1, last=3
```

## Constraints
- 1 ≤ n ≤ 10^5

## Approach
Lower/upper bound variants; be careful with the mid and boundary updates.