struct WaitingNode {
    std::condition_variable cv;
    bool released = false;
};

class Semaphore {
public:
    explicit Semaphore(int count) : count_(count) {
    }

    void Leave() {
        std::unique_lock<std::mutex> lock(mutex_);
        ++count_;
        if (!waiting_queue_.empty()) {
            waiting_queue_.front().released = true;
            waiting_queue_.front().cv.notify_one();
        }
    }

    void Enter() {
        std::unique_lock<std::mutex> lock(mutex_);
        if (count_ == 0 || !waiting_queue_.empty()) {
            waiting_queue_.emplace_back();
            auto& wnode = waiting_queue_.back();
            wnode.cv.wait(lock, [&wnode] { return wnode.released; });
            waiting_queue_.pop_front();
        }
        --count_;
    }

private:
    std::mutex mutex_;
    int count_ = 0;
    std::list<WaitingNode> waiting_queue_;
};