README_EN.md

1115. Print FooBar Alternately

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Description

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

Solutions

Python3

class FooBar:
    def __init__(self, n):
        self.n = n
        self.fooLock = threading.Lock()
        self.barLock = threading.Lock()
        self.barLock.acquire()

    def foo(self, printFoo: 'Callable[[], None]') -> None:
        for i in range(self.n):
            self.fooLock.acquire()
            printFoo()
            self.barLock.release()

    def bar(self, printBar: 'Callable[[], None]') -> None:
        for i in range(self.n):
            self.barLock.acquire()
            printBar()
            self.fooLock.release()

Java

class FooBar {
    private int n;
    private final Semaphore fooSem = new Semaphore(1);
    private final Semaphore barSem = new Semaphore(0);

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

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

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

C++

class FooBar {
private:
    int n;
    mutex fooMu, barMu;

public:
    FooBar(int n) {
        this->n = n;
        barMu.lock();
    }

    void foo(function<void()> printFoo) {
        for (int i = 0; i < n; i++) {
            fooMu.lock();
            printFoo();
            barMu.unlock();
        }
    }

    void bar(function<void()> printBar) {
        for (int i = 0; i < n; i++) {
            barMu.lock();
            printBar();
            fooMu.unlock();
        }
    }
};

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