Variable Window (Sliding Window Sub-pattern)

// Variable window: grow on the right, shrink on the left while invalid.
int longestValidWindow(vector<int>& a) {
    int left = 0, best = 0;
    // state describing the current window (sum, counts, ...)
    for (int right = 0; right < (int)a.size(); right++) {
        add(a[right]);                    // 1. expand: include a[right]
        while (!valid()) {                // 2. shrink until valid again
            remove(a[left]);
            left++;
        }
        best = max(best, right - left + 1); // 3. window [left..right] is valid
    }
    return best;
}