396. Max Sum Subarray Fixed Size

Given an array of integers nums and an integer k, find the maximum sum of any contiguous subarray of size exactly k. You must use the two-pointers or sliding window technique.

Example:
Input: nums = [2, 1, 5, 1, 3, 2], k = 3
Output: 9 (Subarray [5, 1, 3])

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