解决方案

class Foo {

    private AtomicInteger jobDone = new AtomicInteger(0);

    public Foo() {
        
    }

    public void first(Runnable printFirst) throws InterruptedException {
        // printFirst.run() outputs "first".
        printFirst.run();
        // mark the first job as done, by increasing its count.
        jobDone.incrementAndGet();
    }

    public void second(Runnable printSecond) throws InterruptedException {
        while (jobDone.get() != 1);
        // waiting for the first job to be done.
        // printSecond.run() outputs "second". Do not change or remove this line.
        printSecond.run();
        jobDone.incrementAndGet();
    }

    public void third(Runnable printThird) throws InterruptedException {
        while (jobDone.get() != 2);
        // waiting for the first job to be done.
        // printThird.run() outputs "third". Do not change or remove this line.
        printThird.run();
        jobDone.incrementAndGet();
    }
}