Window State: Beyond Sums

Concepts: pyWindowState, pyWindowSet, pyWindowCounter

Sums are the simplest window state. But many sliding window problems require richer state: a set of elements in the window, a frequency counter, a running product, or a boolean condition. The principle is the same: update the state incrementally as elements enter and leave. But the update logic gets more interesting. Window with a Set: Contains Duplicate II (LeetCode 219) Given an array, determine if there are two distinct indices i and j such that arr[i] == arr[j] and |i - j| <= K. Translation: is there a duplicate within any window of size K+1? Maintain a set of elements in the current window. When a new element enters, check if it is already in the set (duplicate found). Then remove the element that is leaving the window. The state is a set, not a sum. The 'add' operation is window.add(