class Foo {
    std::mutex mutexSecond;
    std::mutex mutexThird;
public:
    Foo() {
        mutexSecond.lock();
        mutexThird.lock();
    }

    void first(function<void()> printFirst) {

        // printFirst() outputs "first". Do not change or remove this line.
        printFirst();
        mutexSecond.unlock();
    }

    void second(function<void()> printSecond) {
        std::lock_guard<std::mutex> guard(mutexSecond);
        // printSecond() outputs "second". Do not change or remove this line.
        printSecond();
        mutexThird.unlock();
    }

    void third(function<void()> printThird) {
        std::lock_guard<std::mutex> guard(mutexThird);
        // printThird() outputs "third". Do not change or remove this line.
        printThird();
    }
};