LeetCode-in-Java

1115. Print FooBar Alternately

Medium

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

Solution

import java.util.concurrent.Semaphore;

@SuppressWarnings("java:S2142")
public class FooBar {
    private int n;

    private final Semaphore fooSemaphore;
    private final Semaphore barSemaphore;

    public FooBar(int n) {
        this.n = n;
        fooSemaphore = new Semaphore(1);
        barSemaphore = new Semaphore(1);
        try {
            barSemaphore.acquire();
        } catch (InterruptedException ignored) {
            // ignored
        }
    }

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

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

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