622. Design Circular Queue

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Problem

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.

Implementation the MyCircularQueue class:

• MyCircularQueue(k) Initializes the object with the size of the queue to be k.
• int Front() Gets the front item from the queue. If the queue is empty, return -1.
• int Rear() Gets the last item from the queue. If the queue is empty, return -1.
• boolean enQueue(int value) Inserts an element into the circular queue. Return true if the operation is successful.
• boolean deQueue() Deletes an element from the circular queue. Return true if the operation is successful.
• boolean isEmpty() Checks whether the circular queue is empty or not.
• boolean isFull() Checks whether the circular queue is full or not.

You must solve the problem without using the built-in queue data structure in your programming language.

Example 1:

Input
["MyCircularQueue", "enQueue", "enQueue", "enQueue", "enQueue", "Rear", "isFull", "deQueue", "enQueue", "Rear"]
[[3], [1], [2], [3], [4], [], [], [], [4], []]
Output
[null, true, true, true, false, 3, true, true, true, 4]

Explanation
MyCircularQueue myCircularQueue = new MyCircularQueue(3);
myCircularQueue.enQueue(1); // return True
myCircularQueue.enQueue(2); // return True
myCircularQueue.enQueue(3); // return True
myCircularQueue.enQueue(4); // return False
myCircularQueue.Rear();     // return 3
myCircularQueue.isFull();   // return True
myCircularQueue.deQueue();  // return True
myCircularQueue.enQueue(4); // return True
myCircularQueue.Rear();     // return 4

Constraints:

• 1 <= k <= 1000
• 0 <= value <= 1000
• At most 3000 calls will be made to enQueue, deQueue, Front, Rear, isEmpty, and isFull.

Code

class MyCircularQueue {
    class Node {
        int val;
        Node next;
        Node(int val) {
            this.val = val;
        }
    }

    int size;
    int currSize;
    Node head;
    Node tail;

    public MyCircularQueue(int k) {
        size = k;
        currSize = 0;
    }

    public boolean enQueue(int value) {
        if (currSize == size) return false;

        if (currSize == 0) {
            head = new Node(value);
            tail = head;
        } else {
            tail.next = new Node(value);
            tail = tail.next;
        }

        currSize++;
        return true;
    }

    public boolean deQueue() {
        if (currSize == 0) return false;

        if (currSize == 1) {
            head = null;
            tail = null;
        } else {
            head = head.next;
        }

        currSize--;

        return true;
    }

    public int Front() {
        if (currSize == 0) return -1;

        return head.val;
    }

    public int Rear() {
        if (currSize == 0) return -1;
        
        return tail.val;
    }

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

    public boolean isFull() {
        return currSize == size;
    }
}