设计循环队列详解

设计循环队列的实现。循环队列是一种线性数据结构，其操作基于FIFO(先进先出)原则，最后一个位置与第一个位置连接形成一个圆圈。它也被称为“环形缓冲区”。

循环队列的好处之一是我们可以利用队列前面的空间。在普通队列中，一旦队列满了，即使队列前面有空间，我们也无法插入下一个元素。但是使用循环队列，我们​​可以使用空间来存储新值。

您的实现应支持以下操作：

MyCircularQueue(k)：构造函数，设置队列大小为k。

Front：从队列中获取最前面的项目。如果队列为空，则返回 -1。

Rear：从队列中获取最后一项。如果队列为空，则返回 -1。

enQueue(value): 将一个元素插入循环队列。如果操作成功，则返回 true。

deQueue()：从循环队列中删除一个元素。如果操作成功，则返回 true。

isEmpty()：检查循环队列是否为空。

isFull()：检查循环队列是否已满。

例子：

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()

分析

数组排序实现：

重点是确定循环的空位和满员情况，以及下一个前后标的位置。

一个int length可以记录当前队列的元素个数，和循环周期的大小比较就可以得出是否满，检查长度是否为0，则检测出是否为空。

对后方和前方的下标位置有不同的应用思路：

1.前面代表队列的头部元素位置，代表队列的位置;初始化rear=-1, front=0

2.前面代表队列的头部元素位置，代表队列时可以代表新元素的位置：rear=0, front=0

Tricky不过，对于第一个，读取Front()和Rear()可以直接用front和rear作为下标，对于2，读取Rear()时，需要计算下标：(rear + q.length - 1) % q.length

解决方案

数组实现 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();

数组实现 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 官方解决方案 - 数组实现

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();

使用(双)链表

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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