Print FooBar Alternately.md

Algorithm

Use Semaphores. Here is a good explanation of Semaphores (read their explanation, skip their code).

• use Semaphore fooLock to block execution of foo()
• use Semaphore barLock to block execution of bar()
• fooLock will start unlocked so that foo() is first function to execute.

Solution

class FooBar {
    private int n;
    private Semaphore fooLock = new Semaphore(1);
    private Semaphore barLock = new Semaphore(0);

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

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

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

Time/Space Complexity

• Time Complexity: O(n)
• Space Complexity: O(1)

Links

• github.com/RodneyShag