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.

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.

这题感觉自从大二数据结构以后都没做过。太多coner case，需要重温一下，基本思路就是别妄想用head跟tail就能算出empty或full与否，多开一个size变量，life会容易很多。

class MyCircularQueue {
    int head;
    int tail;
    int[] storage;
    int cap;
    int size;
    /** Initialize your data structure here. Set the size of the queue to be k. */
    public MyCircularQueue(int k) {
        if (k < 1) {
            // Throw exception
        }
        cap = k;
        size = 0;
        storage = new int[k];
        head = -1;
        tail = -1;
    }

    /** Insert an element into the circular queue. Return true if the operation is successful. */
    public boolean enQueue(int value) {
        if (isFull()) {
            return false;
        }
        if (size == 0) {
            head = 0;
            tail = 0;
        } else {
            tail = (tail + 1) % cap;
        }

        storage[tail] = value;


        size++;
        return true;
    }

    /** Delete an element from the circular queue. Return true if the operation is successful. */
    public boolean deQueue() {
        if (isEmpty()) {
            return false;
        }

        head = (head + 1) % cap;        
        size--;
        if (size == 0) {
            head = -1;
            tail = -1;
        }
        return true;
    }

    /** Get the front item from the queue. */
    public int Front() {
        if (isEmpty()) {
            return -1;
        }
        return storage[head];
    }

    /** Get the last item from the queue. */
    public int Rear() {
        if (isEmpty()) {
            return -1;
        }
        return storage[tail];
    }

    /** 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 == cap;
    }
}

/**
 * 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();
 */

哎呀，看了答案，发现还是可以直接head和tail来判断empty和full与否。贴上来参考：

class MyCircularQueue {

    private int[] data;
    private int head;
    private int tail;
    private int size;

    /** Initialize your data structure here. Set the size of the queue to be k. */
    public MyCircularQueue(int k) {
        data = new int[k];
        head = -1;
        tail = -1;
        size = k;
    }

    /** Insert an element into the circular queue. Return true if the operation is successful. */
    public boolean enQueue(int value) {
        if (isFull() == true) {
            return false;
        }
        if (isEmpty() == true) {
            head = 0;
        }
        tail = (tail + 1) % size;
        data[tail] = value;
        return true;
    }

    /** Delete an element from the circular queue. Return true if the operation is successful. */
    public boolean deQueue() {
        if (isEmpty() == true) {
            return false;
        }
        if (head == tail) {
            head = -1;
            tail = -1;
            return true;
        }
        head = (head + 1) % size;
        return true;
    }

    /** Get the front item from the queue. */
    public int Front() {
        if (isEmpty() == true) {
            return -1;
        }
        return data[head];
    }

    /** Get the last item from the queue. */
    public int Rear() {
        if (isEmpty() == true) {
            return -1;
        }
        return data[tail];
    }

    /** Checks whether the circular queue is empty or not. */
    public boolean isEmpty() {
        return head == -1;
    }

    /** Checks whether the circular queue is full or not. */
    public boolean isFull() {
        return ((tail + 1) % size) == head;
    }
}

/**
 * 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();
 */