381. Max Sum Subarray With Fixed Window

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

Example:
Input: nums = [1, 4, 2, 10, 2, 3, 1, 0, 20], k = 3
Output: 21 (from subarray [1, 0, 20])

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