[LeetCode] 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.
• You must solve the problem without using the built-in queue data structure in your programming language.
  Example1:

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

这道题一个比较容易的角度，就是把栈里有多少个元素记住，然后记录出栈和进栈的位置。但是要注意因为需要Front()和Rare()函数，所以需要注意出栈和进栈的时候，不要提前让begin和end++，尤其是end，起始位置要为-1，在begin之后。用count解构这两个index以后，其他的都相对容易，只需要细心

class MyCircularQueue {
    
    int begin;
    int end;
    int count;
    int capacity;
    int[] q;

    public MyCircularQueue(int k) {
        this.q = new int[k];
        this.begin = 0;
        this.end = -1;
        this.count = 0;
        this.capacity = k;
    }
    
    public boolean enQueue(int value) {
        if (isFull()) {
            return false;
        }
        this.count++;
        this.end++;
        this.q[this.end % this.capacity] = value;
        return true;
    }
    
    public boolean deQueue() {
        if (isEmpty()) {
            return false;
        }
        this.count--;
        begin++;
        if (isEmpty()) {
            this.begin = 0;
            this.end = -1;
        }
        return true;
    }
    
    public int Front() {
        if (isEmpty()) {
            return -1;
        }
        return this.q[this.begin % this.capacity];
    }
    
    public int Rear() {
       if (isEmpty()) {
            return -1;
        }
        return this.q[this.end % this.capacity];
    }
    
    public boolean isEmpty() {
        if (this.count == 0) {
            return true;
        }
        return false;
    }
    
    public boolean isFull() {
        if (this.count == this.capacity) {
            return true;
        }
        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();
 */