#include <semaphore.h>

class Foo {
protected:
    sem_t firstJobDone;
    sem_t secondJobDone;
public:
    Foo() {
        sem_init(&firstJobDone, 0, 0);
        sem_init(&secondJobDone, 0, 0);        
    }

    void first(function<void()> printFirst) {
        
        // printFirst() outputs "first". Do not change or remove this line.
        printFirst();
        sem_post(&firstJobDone);
    }

    void second(function<void()> printSecond) {
        sem_wait(&firstJobDone);
        // printSecond() outputs "second". Do not change or remove this line.
        printSecond();
        sem_post(&secondJobDone);
    }

    void third(function<void()> printThird) {
        sem_wait(&secondJobDone);
        // printThird() outputs "third". Do not change or remove this line.
        printThird();
    }
};

运行结果:

Runtime: 144 ms, faster than 62.71% of C++ online submissions for Print in Order.
Memory Usage: 7.1 MB, less than 89.37% of C++ online submissions for Print in Order.