Fixed Window Pattern

For a fixed window of size $k$:

$1.$ Build the initial window with the first $k$ elements.

$2.$ Slide the window: add the next element, remove the first element.

$3.$ Update your answer at each position. Each element enters once and leaves once. Total: $O(n)$. This pattern works for maximum sum of size $k$, average of size $k$, or any aggregate over fixed-size subarrays.