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Formatted question description: https://leetcode.ca/all/1188.html

1188. Design Bounded Blocking Queue

Level

Medium

Description

Implement a thread safe bounded blocking queue that has the following methods:

  • BoundedBlockingQueue(int capacity) The constructor initializes the queue with a maximum capacity.
  • void enqueue(int element) Adds an element to the front of the queue. If the queue is full, the calling thread is blocked until the queue is no longer full.
  • int dequeue() Returns the element at the rear of the queue and removes it. If the queue is empty, the calling thread is blocked until the queue is no longer empty.
  • int size() Returns the number of elements currently in the queue.

Your implementation will be tested using multiple threads at the same time. Each thread will either be a producer thread that only makes calls to the enqueue method or a consumer thread that only makes calls to the dequeue method. The size method will be called after every test case.

Please do not use built-in implementations of bounded blocking queue as this will not be accepted in an interview.

Example 1:

Input:
1
1
["BoundedBlockingQueue","enqueue","dequeue","dequeue","enqueue","enqueue","enqueue","enqueue","dequeue"]
[[2],[1],[],[],[0],[2],[3],[4],[]]

Output:
[1,0,2,2]

Explanation:
Number of producer threads = 1
Number of consumer threads = 1

BoundedBlockingQueue queue = new BoundedBlockingQueue(2);   // initialize the queue with capacity = 2.

queue.enqueue(1);   // The producer thread enqueues 1 to the queue.
queue.dequeue();    // The consumer thread calls dequeue and returns 1 from the queue.
queue.dequeue();    // Since the queue is empty, the consumer thread is blocked.
queue.enqueue(0);   // The producer thread enqueues 0 to the queue. The consumer thread is unblocked and returns 0 from the queue.
queue.enqueue(2);   // The producer thread enqueues 2 to the queue.
queue.enqueue(3);   // The producer thread enqueues 3 to the queue.
queue.enqueue(4);   // The producer thread is blocked because the queue's capacity (2) is reached.
queue.dequeue();    // The consumer thread returns 2 from the queue. The producer thread is unblocked and enqueues 4 to the queue.
queue.size();       // 2 elements remaining in the queue. size() is always called at the end of each test case.

Example 2:

Input:
3
4
["BoundedBlockingQueue","enqueue","enqueue","enqueue","dequeue","dequeue","dequeue","enqueue"]
[[3],[1],[0],[2],[],[],[],[3]]

Output:
[1,0,2,1]

Explanation:
Number of producer threads = 3
Number of consumer threads = 4

BoundedBlockingQueue queue = new BoundedBlockingQueue(3);   // initialize the queue with capacity = 3.

queue.enqueue(1);   // Producer thread P1 enqueues 1 to the queue.
queue.enqueue(0);   // Producer thread P2 enqueues 0 to the queue.
queue.enqueue(2);   // Producer thread P3 enqueues 2 to the queue.
queue.dequeue();    // Consumer thread C1 calls dequeue.
queue.dequeue();    // Consumer thread C2 calls dequeue.
queue.dequeue();    // Consumer thread C3 calls dequeue.
queue.enqueue(3);   // One of the producer threads enqueues 3 to the queue.
queue.size();       // 1 element remaining in the queue.

Since the number of threads for producer/consumer is greater than 1, we do not know how the threads will be scheduled in the operating system, even though the input seems to imply the ordering. Therefore, any of the output [1,0,2] or [1,2,0] or [0,1,2] or [0,2,1] or [2,0,1] or [2,1,0] will be accepted.

Solution

This problem can be solved using semaphores. In the class, create two semaphores semaphoreEnqueue and semaphoreDequeue for methods enqueue() and dequeue() respectively. Also maintain capacity and queue.

For the constructor, initialize capacity using the given capacity, initialize queue, and initialize semaphoreEnqueue to have capacity permits and semaphoreDequeue to have 0 permits.

For method enqueue, acquire a permit from semaphoreEnqueue, offer element to queue and release a permit back to semaphoreDequeue.

For method dequeue, acquire a permit from semaphoreDequeue, poll one element from queue and release a permit back to semaphoreEnqueue. Finally, return the polled element.

For method size, simply return queue.size().

  • class BoundedBlockingQueue {
    int capacity;
    Queue<Integer> queue;
    private Semaphore semaphoreEnqueue;
    private Semaphore semaphoreDequeue;

    public BoundedBlockingQueue(int capacity) {
        this.capacity = capacity;
        queue = new LinkedList<Integer>();
        semaphoreEnqueue = new Semaphore(capacity);
        semaphoreDequeue = new Semaphore(0);
    }
    
    public void enqueue(int element) throws InterruptedException {
        semaphoreEnqueue.acquire();
        queue.offer(element);
        semaphoreDequeue.release();
    }
    
    public int dequeue() throws InterruptedException {
        semaphoreDequeue.acquire();
        int element = queue.poll();
        semaphoreEnqueue.release();
        return element;
    }
    
    public int size() {
        return queue.size();
    }
}

Or, just use key word synchronized to control queue access from different threads.

  • public class Design_Bounded_Blocking_Queue {

    class BoundedBlockingQueue {

        int size;
        Queue<Integer> queue;

        public BoundedBlockingQueue(int capacity) {
            this.queue = new LinkedList<>();
            this.size = capacity;
        }

        public void enqueue(int element) throws InterruptedException {
            synchronized (queue) {
                while (queue.size() == size) {
                    queue.wait();
                }
                queue.offer(element);
                queue.notifyAll();
            }
        }

        public int dequeue() throws InterruptedException {
            synchronized (queue) {
                while (queue.size() == 0) {
                    queue.wait();
                }
                int num = queue.poll();
                queue.notifyAll();
                return num;
            }
        }

        public int size() {
            synchronized (queue) {
                return queue.size();
            }
        }
    }

}

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