Maximum Average Subarray - Fixed Window

Maintain a running sum of $k$ elements. Slide the window by adding the new element and removing the old one.

$1.$ Compute sum of first $k$ elements.

$2.$ For each position from $k$ to $n-1$:

• Add nums[i] (entering the window).
• Subtract nums[i-k] (leaving the window).
• Update max sum if needed.

$3.$ Return max sum divided by $k$.