LeetCode 题解：641. 设计循环双端队列，使用双向链表，JavaScript，详细注释

/**
 * Initialize your data structure here. Set the size of the deque to be k.
 * @param {number} k
 */
function ListNode(value) {
  this.value = value;
  this.prev = this.next = null;
}
var MyCircularDeque = function (k) {
  this.capacity = k; // 储存队列的容量
  this.length = 0; // 储存队列的长度
  const node = this.createNode();
  this.head = node;
  this.tail = node;
};
MyCircularDeque.prototype.createNode = function (value = -1) {
  return new ListNode(value);
};
/**
 * Adds an item at the front of Deque. Return true if the operation is successful.
 * @param {number} value
 * @return {boolean}
 */
MyCircularDeque.prototype.insertFront = function (value) {
  // 判断队列是否已满
  if (this.isFull()) {
    return false;
  }
  // 创建新节点
  const node = this.createNode(value);
  // 将新节点与队首节点相互连接
  this.head.next = node;
  node.prev = this.head;
  // 将头指针指向新队首节点
  this.head = node;
  // 队列长度+1
  this.length++;
  return true;
};
/**
 * Adds an item at the rear of Deque. Return true if the operation is successful.
 * @param {number} value
 * @return {boolean}
 */
MyCircularDeque.prototype.insertLast = function (value) {
  // 判断队列是否已满
  if (this.isFull()) {
    return false;
  }
  // 将新值存储在尾指针的节点上
  this.tail.value = value;
  // 创建新节点
  const node = this.createNode();
  // 将新节点与尾指针的节点相互连接
  this.tail.prev = node;
  node.next = this.tail;
  // 尾指针指向新节点
  this.tail = node;
  // 队列长度+1
  this.length++;
  return true;
};
/**
 * Deletes an item from the front of Deque. Return true if the operation is successful.
 * @return {boolean}
 */
MyCircularDeque.prototype.deleteFront = function () {
  // 判断队列是否为空
  if (this.isEmpty()) {
    return false;
  }
  // 缓存队首的上一个元素
  const temp = this.head.prev;
  // 将队首与其上一个元素的连接互相打断
  this.head.prev = null;
  temp.next = null;
  // 将头指针移动到新队首
  this.head = temp;
  // 队列长度-1
  this.length--;
  return true;
};
/**
 * Deletes an item from the rear of Deque. Return true if the operation is successful.
 * @return {boolean}
 */
MyCircularDeque.prototype.deleteLast = function () {
  // 判断队列是否为空
  if (this.isEmpty()) {
    return false;
  }
  // 缓存队尾元素
  const temp = this.tail.next;
  // 将队尾元素的值设为-1，即删除了队尾的值
  temp.value = -1;
  // 将尾指针与队尾节点的连接打断
  this.tail.next = null;
  temp.prev = null;
  // 将尾指针指向新的插入对位元素的节点
  this.tail = temp;
  // 队列长度-1
  this.length--;
  return true;
};
/**
 * Get the front item from the deque.
 * @return {number}
 */
MyCircularDeque.prototype.getFront = function () {
  // 队首的值可直接返回头节点的值
  return this.head.value;
};
/**
 * Get the last item from the deque.
 * @return {number}
 */
MyCircularDeque.prototype.getRear = function () {
  // 如果队列为空，则返回-1
  if (this.isEmpty()) {
    return -1;
  }
  // 如果队列不为空，则返回头对位的值
  return this.tail.next.value;
};
/**
 * Checks whether the circular deque is empty or not.
 * @return {boolean}
 */
MyCircularDeque.prototype.isEmpty = function () {
  return !this.length;
};
/**
 * Checks whether the circular deque is full or not.
 * @return {boolean}
 */
MyCircularDeque.prototype.isFull = function () {
  return this.length === this.capacity;
};