Rotate an Array by K

Question

## Problem

Rotate an array to the right by `k` steps, in place with O(1) extra space.

## Example
```
[1,2,3,4,5,6,7], k=3 -> [5,6,7,1,2,3,4]
```

## Constraints
- 1 ≤ n ≤ 10^5

## Approach
Reverse-whole, reverse-first-k, reverse-rest. O(n)/O(1).