原文链接: https://leetcode-cn.com/problems/print-foobar-alternately

英文原文

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.

 

Constraints:

• 1 <= n <= 1000

中文题目

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。

通过代码

高赞题解

//手太阴肺经 BLOCKING Queue
public class FooBar {
    private int n;
    private BlockingQueue<Integer> bar = new LinkedBlockingQueue<>(1);
    private BlockingQueue<Integer> foo = new LinkedBlockingQueue<>(1);
    public FooBar(int n) {
        this.n = n;
    }
    public void foo(Runnable printFoo) throws InterruptedException {
        for (int i = 0; i < n; i++) {
            foo.put(i);
            printFoo.run();
            bar.put(i);
        }
    }

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

//手阳明大肠经CyclicBarrier 控制先后
class FooBar6 {
    private int n;

    public FooBar6(int n) {
        this.n = n;
    }

    CyclicBarrier cb = new CyclicBarrier(2);
    volatile boolean fin = true;

    public void foo(Runnable printFoo) throws InterruptedException {
        for (int i = 0; i < n; i++) {
            while(!fin);
            printFoo.run();
            fin = false;
            try {
                cb.await();
            } catch (BrokenBarrierException e) {}
        }
    }

    public void bar(Runnable printBar) throws InterruptedException {
        for (int i = 0; i < n; i++) {
            try {
                cb.await();
            } catch (BrokenBarrierException e) {}
            printBar.run();
            fin = true;
        }
    }
}

//手少阴心经 自旋 + 让出CPU
class FooBar5 {
    private int n;

    public FooBar5(int n) {
        this.n = n;
    }

    volatile boolean permitFoo = true;

    public void foo(Runnable printFoo) throws InterruptedException {     
        for (int i = 0; i < n; ) {
            if(permitFoo) {
        	    printFoo.run();
            	i++;
            	permitFoo = false;
            }else{
                Thread.yield();
            }
        }
    }

    public void bar(Runnable printBar) throws InterruptedException {       
        for (int i = 0; i < n; ) {
            if(!permitFoo) {
        	printBar.run();
        	i++;
        	permitFoo = true;
            }else{
                Thread.yield();
            }
        }
    }
}



//手少阳三焦经 可重入锁 + Condition
class FooBar4 {
    private int n;

    public FooBar4(int n) {
        this.n = n;
    }
    Lock lock = new ReentrantLock(true);
    private final Condition foo = lock.newCondition();
    volatile boolean flag = true;
    public void foo(Runnable printFoo) throws InterruptedException {
        for (int i = 0; i < n; i++) {
            lock.lock();
            try {
            	while(!flag) {
                    foo.await();
                }
                printFoo.run();
                flag = false;
                foo.signal();
            }finally {
            	lock.unlock();
            }
        }
    }

    public void bar(Runnable printBar) throws InterruptedException {
        for (int i = 0; i < n;i++) {
            lock.lock();
            try {
            	while(flag) {
                    foo.await();
            	}
                printBar.run();
                flag = true;
                foo.signal();
            }finally {
            	lock.unlock();
            }
        }
    }
}

//手厥阴心包经 synchronized + 标志位 + 唤醒
class FooBar3 {
    private int n;
    // 标志位，控制执行顺序，true执行printFoo，false执行printBar
    private volatile boolean type = true;
    private final Object foo=  new Object(); // 锁标志

    public FooBar3(int n) {
        this.n = n;
    }
    public void foo(Runnable printFoo) throws InterruptedException {
        for (int i = 0; i < n; i++) {
            synchronized (foo) {
                while(!type){
                    foo.wait();
                }
                printFoo.run();
                type = false;
                foo.notifyAll();
            }
        }
    }

    public void bar(Runnable printBar) throws InterruptedException {
        for (int i = 0; i < n; i++) {
            synchronized (foo) {
                while(type){
                    foo.wait();
                }
                printBar.run();
                type = true;
                foo.notifyAll();
            }
        }
    }
}


//手太阳小肠经 信号量 适合控制顺序
class FooBar2 {
    private int n;
    private Semaphore foo = new Semaphore(1);
    private Semaphore bar = new Semaphore(0);
    public FooBar2(int n) {
        this.n = n;
    }

    public void foo(Runnable printFoo) throws InterruptedException {
        for (int i = 0; i < n; i++) {
            foo.acquire();
        	printFoo.run();
            bar.release();
        }
    }
    public void bar(Runnable printBar) throws InterruptedException {
        for (int i = 0; i < n; i++) {
            bar.acquire();
        	printBar.run();
            foo.release();
        }
    }
}

统计信息

通过次数	提交次数	AC比率
45083	80294	56.1%

提交历史

提交时间	提交结果	执行时间	内存消耗	语言

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