Sliding median (Two Heaps Sub-pattern)

// Sliding window median with two heaps and lazy deletion.
vector<double> medianSlidingWindow(vector<int>& nums, int k) {
    priority_queue<int> low;                                // smaller half
    priority_queue<int, vector<int>, greater<int>> high;   // larger half
    unordered_map<int, int> delayed;                        // value -> copies waiting to be removed
    int lowSize = 0, highSize = 0;                          // LIVE counts only
    auto prune = [&](auto& h) {                             // drop stale tops
        while (!h.empty() && delayed.count(h.top())) {
            if (--delayed[h.top()] == 0) delayed.erase(h.top());
            h.pop();
        }
    };
    auto rebalance = [&]() {                                // low has the same size or one more
        if (lowSize > highSize + 1) {
            high.push(low.top()); low.pop(); lowSize--; highSize++; prune(low);
        } else if (lowSize < highSize) {
            low.push(high.top()); high.pop(); highSize--; lowSize++; prune(high);
        }
    };
    vector<double> res;
    for (int i = 0; i < (int)nums.size(); i++) {
        int x = nums[i];
        if (low.empty() || x <= low.top()) { low.push(x); lowSize++; }
        else { high.push(x); highSize++; }
        if (i >= k) {                                       // nums[i-k] leaves the window
            int gone = nums[i - k];
            delayed[gone]++;
            if (gone <= low.top()) { lowSize--; if (gone == low.top()) prune(low); }
            else { highSize--; if (gone == high.top()) prune(high); }
        }
        rebalance();
        if (i >= k - 1)
            res.push_back(k % 2 ? (double)low.top() : ((double)low.top() + high.top()) / 2.0);
    }
    return res;
}