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622. 设计循环队列


622. 设计循环队列

🟠   🔖  设计 队列 数组 链表  🔗 力扣open in new window LeetCodeopen in new window

题目

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.

Example 1:

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

Constraints:

  • 1 <= k <= 1000
  • 0 <= value <= 1000
  • At most 3000 calls will be made to enQueue, deQueue, Front, Rear, isEmpty, and isFull.

题目大意

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。

解题思路

  • 设计一个环形队列,底层用数组实现;
  • 额外维护 4 个变量,队列的总 size ,队头下标 head ,队尾下标 tail ;
  • 每此入队和出队都更新 head , tail ,下标需要对 size 取余,因为超过 size 大小之后,需要循环存储;

代码

/**
 * @param {number} k
 */
var MyCircularQueue = function (k) {
	this.queue = new Array(k);
	this.head = 0;
	this.tail = 0;
	this.size = k;
};

/**
 * @param {number} value
 * @return {boolean}
 */
MyCircularQueue.prototype.enQueue = function (value) {
	if (this.isFull()) return false;
	this.queue[this.tail] = value;
	// 将 tail 指向队尾的下一个空间
	this.tail = (this.tail + 1) % this.size;
	return true;
};

/**
 * @return {boolean}
 */
MyCircularQueue.prototype.deQueue = function () {
	if (this.isEmpty()) return false;
	this.queue[this.head] = null;
	// 将 head 指向新的队头
	this.head = (this.head + 1) % this.size;
	return true;
};

/**
 * @return {number}
 */
MyCircularQueue.prototype.Front = function () {
	return this.isEmpty() ? -1 : this.queue[this.head];
};

/**
 * @return {number}
 */
MyCircularQueue.prototype.Rear = function () {
	// 因为 tail 指向队尾的下一个空间,所以要分情况处理
	// 若 tail == 0,那队尾应该在 size - 1
	// 其他情况,队尾在 tail - 1
	let index = this.tail === 0 ? this.size - 1 : this.tail - 1;
	return this.isEmpty() ? -1 : this.queue[index];
};

/**
 * @return {boolean}
 */
MyCircularQueue.prototype.isEmpty = function () {
	// head == tail 时,有两种情况
	// 一种是队列为空,此时 queue[head] 为null
	// 一种是队列满了,此时 queue[head] 有值
	return this.head === this.tail && !this.queue[this.head];
};

/**
 * @return {boolean}
 */
MyCircularQueue.prototype.isFull = function () {
	return this.head === this.tail && !!this.queue[this.head];
};

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