双端队列跟队列的区别就是：队列只有一头进一头出，而双端队列两头都可以进可以出。灵活性变高，同样的也相对复杂。

    1. class Deque {
    2.     constructor() {
    3.         this.count = 0;
    4.         this.lowestCount = 0;
    5.         this.items = {};
    6.     }
    8.     /**
    9.      * 向队列前端添加元素
    10.      * @param {*} element 
    11.      */
    12.     addFront(element) {
    13.         if (this.isEmpty()) {
    14.             this.addBack(element);
    15.         } else if (this.lowestCount > 0) {
    16.             this.lowestCount--;
    17.             this.items[this.lowestCount] = element;
    18.         } else {
    19.             for (let i = this.count; i > 0; i--) {
    20.                 this.items[i] = this.items[i - 1];
    21.             }
    23.             this.count++;
    24.             this.items[0] = element;
    25.         }
    26.     }
    28.     /**
    29.      * 向队尾添加新的项
    30.      * @param {*} element 
    31.      */
    32.     addBack(element) {
    33.         this.items[this.count] = element;
    34.         this.count++;
    35.     }
    37.     /**
    38.      * 移除队列第一项
    39.      */
    40.     removeFront() {
    41.         if (this.isEmpty()) {
    42.             return;
    43.         }
    45.         let element = this.items[this.lowestCount];
    46.         delete this.items[this.lowestCount];
    47.         this.lowestCount++;
    48.         return element;
    49.     }
    51.     /**
    52.      * 移除队列最后一项
    53.      */
    54.     removeBack() {
    55.         if (this.isEmpty()) {
    56.             return;
    57.         }
    59.         this.count--;
    60.         let element = this.items[this.count];
    61.         delete this.items[this.count];
    62.         return element;
    63.     }
    65.     /**
    66.      * 返回队列第一项
    67.      */
    68.     peekFront() {
    69.         return this.isEmpty() ? null : this.items[this.lowestCount];
    70.     }
    72.     /**
    73.      * 返回队列最后一项
    74.      */
    75.     peekBack() {
    76.         return this.isEmpty() ? null : this.items[this.count];
    77.     }
    79.     isEmpty() {
    80.         return this.size() === 0;
    81.     }
    83.     size() {
    84.         return this.count - this.lowestCount;
    85.     }
    87.     clear() {
    88.         this.count = 0;
    89.         this.lowestCount = 0;
    90.         this.items = {};
    91.     }
    93.     toString() {
    94.         if (this.isEmpty()) {
    95.             return '';
    96.         }
    98.         let str = this.items[this.lowestCount].toString();
    100.         for (let i = this.lowestCount + 1; i < this.count; i++) {
    101.             str += `,${this.items[i]}`;
    102.         }
    104.         return str;
    105.     }
    106. }