622. 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.

Implement 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.

Solution:

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

    ListNode head;
    ListNode tail;
    int size;
    int capacity;

    public MyCircularQueue(int k) {
        this.capacity = k;
        this.size = 0;
        head = new ListNode(-1);
        tail = new ListNode(-1);

        head.next = tail;
        tail.prev = head;
    }
    // fifo
    // head -> - >tail
    // 

    public boolean enQueue(int value) {
        if (isFull()){
            return false;
        }

        ListNode newNode = new ListNode(value);
        newNode.next = head.next;
        newNode.prev = head;
        head.next.prev = newNode;
        head.next = newNode;
        size++;
        return true;
    }

    public boolean deQueue() {
        if (isEmpty()){
            return false;
        }

        ListNode last = tail.prev;
        last.prev.next = tail;
        tail.prev = last.prev;
        size--;
        return true;
    }

    public int Front() {
        if (isEmpty()){
            return -1;
        }else{
            return tail.prev.val;
        }
    }

    public int Rear() {
        if (isEmpty()){
            return -1;
        }else{
            return head.next.val;
        }
    }

    public boolean isEmpty() {
        if (size == 0){
            return true;
        }else{
            return false;
        }
    }

    public boolean isFull() {
        if (size == capacity){
            return true;
        }else{
            return false;
        }
    }
}

/**
 * Your MyCircularQueue object will be instantiated and called as such:
 * MyCircularQueue obj = new MyCircularQueue(k);
 * boolean param_1 = obj.enQueue(value);
 * boolean param_2 = obj.deQueue();
 * int param_3 = obj.Front();
 * int param_4 = obj.Rear();
 * boolean param_5 = obj.isEmpty();
 * boolean param_6 = obj.isFull();
 */


 // TC: O(1)
 // SC: O(1)