622. Design Circular Queue

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.

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.

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.

Follow up: Could you solve the problem without using the built-in queue?

My Solutions:

所谓的circular queue就是指有两个指针front和rear分别指向这个queue的第一个元素和最后一个元素，用一个int variable length记录queue实际含有几个元素。

当enqueue一个元素，需要把rear指向这个新进入的元素所在的index。如果queue已经满了，则不能继续加元素。由于这个queue是首尾相连的，当queue不为空的时候，元素加到rear的index上，但rear可能回到array的前部，所以需要用 (rear + 1) % queue.length决定rear的新位置。

当dequeue一个元素，需要把front指向当前front的下一个index。front的新位置同理。

class MyCircularQueue {
    
    private int[] queue;
    private int front, rear;
    private int length;

    public MyCircularQueue(int k) {
        this.queue = new int[k];
        this.front = 0;
        this.rear = -1;
        this.length = 0;
    }
    
    public boolean enQueue(int value) {
        if (isFull()) return false;
        rear = (rear + 1) % queue.length;
        queue[rear] = value;
        length++;
        return true;
    }
    
    public boolean deQueue() {
        if (isEmpty()) return false;
        front = (front + 1) % queue.length;
        length--;
        return true;
    }
    
    public int Front() {
        return isEmpty() ? -1 : queue[front];
    }
    
    public int Rear() {
        return isEmpty() ? -1 : queue[rear];
    }
    
    public boolean isEmpty() {
        return length == 0;
    }
    
    public boolean isFull() {
        return length == queue.length;
    }
}