1115. 交替打印FooBar

题目

我们提供了一个类：

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。

解法

解法一：

（超时）

while循环死等

JAVA

class FooBar {

		private transient int flag = 1;

		private int n;

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

		public void foo(Runnable printFoo) throws InterruptedException {
			for (int i = 0; i < n; i++) {
				while (true) {
					if (1 == flag) {
						break;
					}
				}
				// printFoo.run() outputs "foo". Do not change or remove this
				// line.
				printFoo.run();
				flag = 2;
			}
		}

		public void bar(Runnable printBar) throws InterruptedException {

			for (int i = 0; i < n; i++) {
				while (true) {
					if (2 == flag) {
						break;
					}
				}
				// printBar.run() outputs "bar". Do not change or remove this
				// line.
				printBar.run();
				flag = 1;
			}
		}
	}

解法二：

使用Condition做精准唤醒

Java

class FooBar {
		
		private int flag = 1;
		
		ReentrantLock lock = new ReentrantLock();
		Condition c1 = lock.newCondition();
		Condition c2 = lock.newCondition();
		
	    private int n;

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

	    public void foo(Runnable printFoo) throws InterruptedException {
	        try {
	        	lock.lock();
		        
		        for (int i = 0; i < n; i++) {
		        	while (1 != flag) {
			        	c1.await();
			        }
		        	// printFoo.run() outputs "foo". Do not change or remove this line.
		        	printFoo.run();
		        	flag = 2;
		        	c2.signal();
		        }
	        } finally {
	        	lock.unlock();
	        }
	    }

	    public void bar(Runnable printBar) throws InterruptedException {
	        
	        try {
	        	lock.lock();
	        	for (int i = 0; i < n; i++) {
	        		while (2 != flag) {
			        	c2.await();
			        }
		            // printBar.run() outputs "bar". Do not change or remove this line.
		        	printBar.run();
		        	flag = 1;
	        		c1.signal();
		        }
	        } finally {
	        	lock.unlock();
	        }
	    }
	}

解法三：

使用wait和notify

Java

class FooBar {

		private Object lock = new Object();

		private int flag = 1;

		private int n;

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

		public void foo(Runnable printFoo) throws InterruptedException {
			synchronized (lock) {
				for (int i = 0; i < n; i++) {
					if (1 != flag) {
						lock.wait();
					}
					// printFoo.run() outputs "foo". Do not change or remove
					// this
					// line.
					printFoo.run();
					flag = 2;
					lock.notifyAll();
				}
			}
		}

		public void bar(Runnable printBar) throws InterruptedException {

			synchronized (lock) {
				for (int i = 0; i < n; i++) {
					if (2 != flag) {
						lock.wait();
					}
					// printBar.run() outputs "bar". Do not change or remove
					// this
					// line.
					printBar.run();
					flag = 1;
					lock.notifyAll();
				}
			}

		}
	}

解法四：

使用CyclicBarrier

class FooBar {
    private int n;

    public FooBar(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;
        }
    }
}

解法五：

使用Semaphore

class FooBar {

		private Semaphore s1 = new Semaphore(0);
		private Semaphore s2 = new Semaphore(1);

		private int n;

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

		public void foo(Runnable printFoo) throws InterruptedException {
			for (int i = 0; i < n; i++) {
				// printFoo.run() outputs "foo". Do not change or remove
				// this
				// line.
				s2.acquire();
				printFoo.run();
				s1.release();
			}
		}

		public void bar(Runnable printBar) throws InterruptedException {
			
			for (int i = 0; i < n; i++) {
				// printBar.run() outputs "bar". Do not change or remove
				// this
				// line.
				s1.acquire();
				printBar.run();
				s2.release();
			}
		}
	}