题目

英文

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.

中文

设计你的循环队列实现。循环队列是一种线性数据结构，其操作表现基于 FIFO（先进先出）原则并且队尾被连接在队首之后以形成一个循环。它也被称为“环形缓冲器”。

循环队列的一个好处是我们可以利用这个队列之前用过的空间。在一个普通队列里，一旦一个队列满了，我们就不能插入下一个元素，即使在队列前面仍有空间。但是使用循环队列，我们能使用这些空间去存储新的值。

你的实现应该支持如下操作：

MyCircularQueue(k): 构造器，设置队列长度为 k 。
Front: 从队首获取元素。如果队列为空，返回 -1 。
Rear: 获取队尾元素。如果队列为空，返回 -1 。
enQueue(value): 向循环队列插入一个元素。如果成功插入则返回真。
deQueue(): 从循环队列中删除一个元素。如果成功删除则返回真。
isEmpty(): 检查循环队列是否为空。
isFull(): 检查循环队列是否已满。

解法

官方解法-更符合题意

class MyCircularQueue {

  private int[] queue;
  private int count,front,rear;

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

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

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

  /** Get the last item from the queue. */
  public int Rear() {
    if (isEmpty()) return  -1;
    return rear == 0 ? queue[queue.length - 1] : queue[rear - 1];
  }

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

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

解法二：利用数组长度和计数降低难度

class MyCircularQueue {

  private int[] queue;
  private int count,front,rear;

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

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

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

  /** Get the last item from the queue. */
  public int Rear() {
    if (isEmpty()) return  -1;
    return rear == 0 ? queue[queue.length - 1] : queue[rear - 1];
  }

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

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