622. Design Circular Queue Medium 1513158Add to ListShare

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.

解法：

1.如何判定队列是满的，如何判定队列是空的？ 很巧妙的增加了一个空位来确保

2.rear和front每次++后要取模保证永远在界内

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