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.

Analysis

Array 数组实现:

重点在于确定循环队列空和满的情况,以及确定下一个rear和front的下标位置。

设定一个 int length 可以记录当前queue的元素个数,和循环队列的大小比较就可以得到是否满,检查length是否为0,则检测队列是否为空。

对于rear和front的下标位置有两种可行思路:

  1. front代表queue的头部元素位置,rear代表queue的尾部元素位置;初始化rear=-1, front=0
  2. front代表queue的头部元素位置,rear代表queue的尾部可以填充新元素的位置;初始化时:rear=0, front=0

Tricky之处在于对于1,读取Front()和Rear()可以直接用front和rear作为下标,但是对于2,读取Rear()时,需要计算下标:(rear + q.length - 1) % q.length

Solution

Array Implementation 1 - init front = 0, rear = -1

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

    /** Initialize your data structure here. Set the size of the queue to be k. */
    public MyCircularQueue(int k) {
        q = new int[k];
        length = 0;
        front = 0;
        rear = -1;
    }

    /** Insert an element into the circular queue. Return true if the operation is successful. */
    public boolean enQueue(int value) {
        if (isFull()) {
            return false;
        }
        rear = (rear + 1) % (q.length);
        q[rear] = value;
        length++;
        return true;
    }

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

    /** Get the front item from the queue. */
    public int Front() {
        return isEmpty() ? -1 : q[front];
    }

    /** Get the last item from the queue. */
    public int Rear() {
        return isEmpty() ? -1 : q[rear];
    }

    /** Checks whether the circular queue is empty or not. */
    public boolean isEmpty() {
        return length == 0;
    }

    /** Checks whether the circular queue is full or not. */
    public boolean isFull() {
        return length == q.length;
    }
}

/**
 * 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();
 */

Array Implementation 2 - init front = 0, rear = 0

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

    /** Initialize your data structure here. Set the size of the queue to be k. */
    public MyCircularQueue(int k) {
        q = new int[k];
        length = 0;
        front = 0;
        rear = 0;
    }

    /** Insert an element into the circular queue. Return true if the operation is successful. */
    public boolean enQueue(int value) {
        if (isFull()) {
            return false;
        }
        q[rear] = value;
        rear = (rear + 1) % (q.length);
        length++;
        return true;
    }

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

    /** Get the front item from the queue. */
    public int Front() {
        return isEmpty() ? -1 : q[front];
    }

    /** Get the last item from the queue. */
    public int Rear() {
        return isEmpty() ? -1 : q[(rear + q.length - 1) % q.length];
    }

    /** Checks whether the circular queue is empty or not. */
    public boolean isEmpty() {
        return length == 0;
    }

    /** Checks whether the circular queue is full or not. */
    public boolean isFull() {
        return length == q.length;
    }
}

/**
 * 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();
 */

LeetCode Official Solution - Array Implementation

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();
 */

Using (Doubly) Linked List

class ListNode {
    int val;
    ListNode prev, next;
    public ListNode(int x) {
        val = x;
        prev = null;
        next = null;
    }
}

class MyCircularQueue {
    int queueSize, currSize;
    ListNode head, tail;

    /** Initialize your data structure here. Set the size of the queue to be k. */
    public MyCircularQueue(int k) {
        queueSize = k;
        currSize = 0;
        head = new ListNode(-1);
        tail = new ListNode(-1);
        head.next = tail;
        tail.prev = head;
    }

    /** Insert an element into the circular queue. Return true if the operation is successful. */
    public boolean enQueue(int value) {
        if (isFull()) {
            return false;
        }
        ListNode newNode = new ListNode(value);
        newNode.next = tail;
        newNode.prev = tail.prev;
        tail.prev.next = newNode;
        tail.prev = newNode;
        currSize++;
        return true;
    }

    /** Delete an element from the circular queue. Return true if the operation is successful. */
    public boolean deQueue() {
        if (isEmpty()) {
            return false;
        }
        ListNode toBeDeleted = head.next;
        head.next = toBeDeleted.next;
        toBeDeleted.next.prev = head;
        toBeDeleted.next = null;
        toBeDeleted.prev = null;
        currSize--;
        return true;
    }

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

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

    /** Checks whether the circular queue is empty or not. */
    public boolean isEmpty() {
        return currSize == 0;
    }

    /** Checks whether the circular queue is full or not. */
    public boolean isFull() {
        return currSize == queueSize;
    }
}

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