Given an integer array `arr` and an integer `k`, find the **maximum sum** of any contiguous subarray of size `k`. Use a sliding window of fixed length `k` to compute each subarray sum efficiently.
arr = array of integers, k = subarray size
integer (maximum subarray sum of size k)
Example 1:
Input:
arr = [1,4,2,10,23,3,1,0,20] k = 4
Output:
39
Example 2:
Input:
arr = [2,3,4] k = 2
Output:
7