[Java]LeetCode1115. 交替打印FooBar | Print FooBar Alternately

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Suppose you are given the following code:

class FooBar {
public void foo() {
    for (int i = 0; i < n; i++) {
      print("foo");
  }
}

public void bar() {
    for (int i = 0; i < n; i++) {
      print("bar");
    }
}
}
The same instance of FooBar will be passed to two different threads. Thread A will call foo() while thread B will call bar(). Modify the given program to output "foobar" n times.

Example 1:

Input: n = 1
Output: "foobar"
Explanation: There are two threads being fired asynchronously. One of them calls foo(), while the other calls bar(). "foobar" is being output 1 time.
Example 2:

Input: n = 2
Output: "foobarfoobar"
Explanation: "foobar" is being output 2 times.

---

我们提供一个类：

class FooBar {
public void foo() {
    for (int i = 0; i < n; i++) {
      print("foo");
  }
}

public void bar() {
    for (int i = 0; i < n; i++) {
      print("bar");
    }
}
}
两个不同的线程将会共用一个 FooBar 实例。其中一个线程将会调用 foo() 方法，另一个线程将会调用 bar() 方法。

请设计修改程序，以确保 "foobar" 被输出 n 次。

示例 1:

输入: n = 1
输出: "foobar"
解释: 这里有两个线程被异步启动。其中一个调用 foo() 方法, 另一个调用 bar() 方法，"foobar" 将被输出一次。
示例 2:

输入: n = 2
输出: "foobarfoobar"
解释: "foobar" 将被输出两次。

---

15ms

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

class FooBar {
    private int n;
    private boolean fooFlag;
    private boolean barFlag;
    private Lock lock;
    private Condition fooCondition;
    private Condition barCondition;

    public FooBar(int n) {
        this.n = n;
        this.fooFlag = false;
        this.barFlag = true;
        this.lock = new ReentrantLock();
        this.fooCondition = lock.newCondition();
        this.barCondition = lock.newCondition();
    }

    public void foo(Runnable printFoo) throws InterruptedException {
        for (int i = 0; i < n; i++) {
            lock.lock();

            try {
                while (fooFlag) {
                    fooCondition.await();
                }

                /**
                 * printFoo.run() outputs "foo". Do not change or remove this line.
                 */
                printFoo.run();
                fooFlag = true;
                barFlag = false;
                barCondition.signal();
            } finally {
                lock.unlock();
            }
        }
    }

    public void bar(Runnable printBar) throws InterruptedException {
        for (int i = 0; i < n; i++) {
            lock.lock();

            try {
                while (barFlag) {
                    barCondition.await();
                }

                /**
                 * printBar.run() outputs "bar". Do not change or remove this line.
                 */
                printBar.run();
                barFlag = true;
                fooFlag = false;
                fooCondition.signal();
            } finally {
                lock.unlock();
            }
        }
    }
}

---

16ms

import java.util.concurrent.Semaphore;

class FooBar {
    private int n;
    private Semaphore semaphore = new Semaphore(1);
    private Semaphore semaphore2 = new Semaphore(0);
    public FooBar(int n) {
        this.n = n;
    }

    public void foo(Runnable printFoo) throws InterruptedException {
        
        for (int i = 0; i < n; i++) {
            semaphore.acquire();
            // printFoo.run() outputs "foo". Do not change or remove this line.
            printFoo.run();
            semaphore2.release();
        }
    }

    public void bar(Runnable printBar) throws InterruptedException {
        
        for (int i = 0; i < n; i++) {
            semaphore2.acquire();
            // printBar.run() outputs "bar". Do not change or remove this line.
            printBar.run();
            semaphore.release();
        }
    }
}

---

24ms

class FooBar {
    private int n;
    private static final Object LOCK = new Object();
    public FooBar(int n) {
        this.n = n;
    }

    public void foo(Runnable printFoo) throws InterruptedException {
        for (int i = 0; i < n; i++) {
            synchronized (LOCK) {
                LOCK.notify();
                printFoo.run();
                LOCK.wait();
            }
        }
    }

    public void bar(Runnable printBar) throws InterruptedException {
        for (int i = 0; i < n; i++) {
            synchronized (LOCK) {
                LOCK.notify();
                printBar.run();
                if (i < n - 1) {
                    LOCK.wait();
                }
            }
        }
    }
}