Design Circular Queue

Design your implementation of the circular queue. The circular queue is a linear data structure in which the operations are performed based on FIFO (First In First Out) principle and the last position is connected back to the first position to make a circle. It is also called "Ring Buffer".

One of the benefits of the circular queue is that we can make use of the spaces in front of the queue. In a normal queue, once the queue becomes full, we cannot insert the next element even if there is a space in front of the queue. But using the circular queue, we can use the space to store new values.

Your implementation should support following operations:

• MyCircularQueue(k): Constructor, set the size of the queue to be k.

• Front: Get the front item from the queue. If the queue is empty, return -1.

• Rear: Get the last item from the queue. If the queue is empty, return -1.

• enQueue(value): Insert an element into the circular queue. Return true if the operation is successful.

• deQueue(): Delete an element from the circular queue. Return true if the operation is successful.

• isEmpty(): Checks whether the circular queue is empty or not.

• isFull(): Checks whether the circular queue is full or not.

Example

MyCircularQueue circularQueue = new MyCircularQueue(3); // set the size to be 3
circularQueue.enQueue(1);  // return true
circularQueue.enQueue(2);  // return true
circularQueue.enQueue(3);  // return true
circularQueue.enQueue(4);  // return false, the queue is full
circularQueue.Rear();  // return 3
circularQueue.isFull();  // return true
circularQueue.deQueue();  // return true
circularQueue.enQueue(4);  // return true
circularQueue.Rear();  // return 4

Note

本质有点类似双端队列，所以可以用数组实现或者链表实现

注意一下特殊情况，比如deQueue的时候，如果是最后一个元素，front和rear都要设为-1

Code

class MyCircularQueue {
    final int[] a;
    int front, rear = -1, len = 0;

    public MyCircularQueue(int k) { a = new int[k];}

    public boolean enQueue(int val) {
        if (!isFull()) {
            rear = (rear + 1) % a.length;
            a[rear] = val;
            len++;
            return true;
        } else return false;
    }

    public boolean deQueue() {
        if (!isEmpty()) {
            front = (front + 1) % a.length;
            len--;
            return true;
        } else return false;
    }

    public int Front() { return isEmpty() ? -1 : a[front];}

    public int Rear() {return isEmpty() ? -1 : a[rear];}

    public boolean isEmpty() { return len == 0;}

    public boolean isFull() { return len == a.length;}
}

class MyCircularQueue {

    LinkedList<Integer> q;
    int cap;
    int front = -1;
    int rear = -1;
    /** Initialize your data structure here. Set the size of the queue to be k. */
    public MyCircularQueue(int k) {
        this.cap = k;
        this.q = new LinkedList<>();
    }

    /** Insert an element into the circular queue. Return true if the operation is successful. */
    public boolean enQueue(int value) {
        if (isFull()) {
            return false;
        }
        if (isEmpty()) {
            front = value; 
        }
        q.add(value);
        rear = value;
        return true;
    }

    /** Delete an element from the circular queue. Return true if the operation is successful. */
    public boolean deQueue() {
        if (isEmpty()) {
            return false;
        }

        q.removeFirst();
        if (isEmpty()) {
            front = -1;
            rear = -1;
        } else {
            front = q.get(0);
        }
        return true;
    }

    /** Get the front item from the queue. */
    public int Front() {
        return front;
    }

    /** Get the last item from the queue. */
    public int Rear() {
        return rear;
    }

    /** Checks whether the circular queue is empty or not. */
    public boolean isEmpty() {
        return q == null || q.size() == 0;
    }

    /** Checks whether the circular queue is full or not. */
    public boolean isFull() {
        return q.size() == cap;
    }
}