Maximum Sum Subarray of Size K

Question

## Problem

Find the maximum sum of any contiguous subarray of length `k`.

## Example
```
[2,1,5,1,3,2], k=3 -> 9
```

## Constraints
- 1 ≤ k ≤ n ≤ 10^5

## Approach
Fixed-size sliding window. O(n).