LeetCode 622 - Design Circular Queue

https://www.cnblogs.com/grandyang/p/9899034.html

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:

• All values will be in the range of [0, 1000].
• The number of operations will be in the range of [1, 1000].
• Please do not use the built-in Queue library.

https://zxi.mytechroad.com/blog/uncategorized/leetcode-622-design-circular-queue/
https://leetcode.com/problems/design-circular-queue/discuss/149420/Concise-Java-using-array

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;}
}

https://leetcode.com/problems/design-circular-queue/discuss/175023/Java-Solution-with-Explanation

I keep variables for capacity, size, and the front and back pointer. I start from index 0 for the front and back pointer and increment as they are used. This way, the implementation is identical to how I can implement a deque, but since it's circular I % it by the array size.

Back starts at -1, since the only time we will use back is on the enque, and I will increment it before accessing it. Front can start at 0, since it will just only be used for dequeing. I could make it shorter by removing the size variable, but it was quicker to come up with this solution.
size is incremented on enqueue and decremented on deque.

On every enque we increment the back pointer since the list will get bigger, and store the value.
On every deque we increment the front pointer since the list will get smaller (makes the gap between back and front smaller).

Rear will be the element at the back pointer% length
Front will be the element at the front pointer %length

class MyCircularQueue {
    int [] arr;
    int size;
    int capacity;
    int front;
    int back;
    /** Initialize your data structure here. Set the size of the queue to be k. */
    public MyCircularQueue(int k) {
        arr = new int[k];
        capacity=k;
        size=0;
        front=0;
        back = -1;
    }
    
    /** Insert an element into the circular queue. Return true if the operation is successful. */
    public boolean enQueue(int value) {
        if(size==capacity){
            return false;
        }
        ++back;
        arr[back%arr.length]=value;
        ++size;
        return true;
    }
    
    /** Delete an element from the circular queue. Return true if the operation is successful. */
    public boolean deQueue() {
        if(size==0)return false;
        ++front;
        --size;
        return true;
    }
    
    /** Get the front item from the queue. */
    public int Front() {
        if(size==0)return -1;
        return arr[front%arr.length];
    }
    
    /** Get the last item from the queue. */
    public int Rear() {
        if(size==0)return-1;
        return arr[back%arr.length];
    }
    
    /** Checks whether the circular queue is empty or not. */
    public boolean isEmpty() {
        return size==0;
    }
    
    /** Checks whether the circular queue is full or not. */
    public boolean isFull() {
        return size==capacity;
    }
}