leetcode 1114：按需打印

解法一：Semaphore

class Foo {
    private Semaphore first = new Semaphore(1);
    private Semaphore second = new Semaphore(0);
    private Semaphore third = new Semaphore(0);

    public Foo() {
        
    }

    public void first(Runnable printFirst) throws InterruptedException {
        
        // printFirst.run() outputs "first". Do not change or remove this line.
        first.acquire();
        printFirst.run();
        second.release();
    }

    public void second(Runnable printSecond) throws InterruptedException {
        
        // printSecond.run() outputs "second". Do not change or remove this line.
        second.acquire();
        printSecond.run();
        third.release();
    }

    public void third(Runnable printThird) throws InterruptedException {
        
        // printThird.run() outputs "third". Do not change or remove this line.
        third.acquire();
        printThird.run();
        first.release();
    }
}

解法二 CountDownLatch

class Foo {
    private CountDownLatch second = new CountDownLatch(1);
    private CountDownLatch third = new CountDownLatch(1);

    public Foo() {
        
    }

    public void first(Runnable printFirst) throws InterruptedException {
        
        // printFirst.run() outputs "first". Do not change or remove this line.
        printFirst.run();
        second.countDown();
    }

    public void second(Runnable printSecond) throws InterruptedException {
        
        // printSecond.run() outputs "second". Do not change or remove this line.
        second.await();
        printSecond.run();
        third.countDown();
    }

    public void third(Runnable printThird) throws InterruptedException {
        
        // printThird.run() outputs "third". Do not change or remove this line.
        third.await();
        printThird.run();
    }
}

解法三 synchronized

class Foo {
    private    boolean first = false;
    private    boolean second = false;
    private static  Object lock = new Object();


    public Foo() {
        
    }

    public  void first(Runnable printFirst) throws InterruptedException {
        synchronized (lock) {
            // printFirst.run() outputs "first". Do not change or remove this line.
            printFirst.run();
            first = true;
            lock.notifyAll();
        }
    }

    public void second(Runnable printSecond) throws InterruptedException {
        synchronized (lock) {
            while (!first) {
                lock.wait();
            }
            // printSecond.run() outputs "second". Do not change or remove this line.
            printSecond.run();  
            second = true;
            lock.notifyAll();
        }
    }

    public void third(Runnable printThird) throws InterruptedException {
        synchronized (lock) {
            while (!second) {
                lock.wait();
            }
            // printThird.run() outputs "third". Do not change or remove this line.
            printThird.run();
        }

    }
}