栈，队列，双端队列

栈

遵循后进先出原则的有序集合。新添加的元素或待删除的元素都保存在栈的同一端，称为栈顶，另一端为栈底，在栈中，新元素都靠近栈顶，旧元素都靠近栈底

class Stack{
    constructor(){
        this.count = 0
        this.items = []
    }
    push(element){     // 插入元素
        this.items[this.count] = element
        this.count ++
    }
    isEmpty(){      // 判断是否为空
        return this.count === 0
    }
    size(){     // 数组长度
        return this.count
    }
    pop(){      // 删除栈顶元素
        if(isEmpty()){
            return undefined
        }
        this.count--
        const result = this.items[this.count]
        delete items[this.count]
        return result
    }
    peek(){     // 查看栈顶元素
        if(isEmpty()){
            return undefined
        }
        return this.items[this.count-1]
    }
    clear(){    // 清空栈
        this.items = []
        this.count = 0
    }
    toString(){     // toString方法
        if(isEmpty()){
            return ""
        }
        let objString = `${this.items = [0]}`
        for(let i = 1; i < this.count; i++){
            objString = `${objString},${this.items[i]}`
        }
        return objString
    }
}

队列

遵循先进先出原则

class Queue{
    // 队列
    constructor(){
        this.count = 0  // 长度
        this.lowestCount = 0    // 头元素索引
        this.items = {}
    }
    enqueue(element){   // 向队列中添加元素，添加到队列末端
        this.items[this.count] = element
        this.count ++
    }
    isEmpty(){
        return this.count===0
    }
    dequeue(){      // 移除队列中的头元素
        if(isEmpty()){
            return undefined
        }
        const result = this.items[this.lowestCount]
        delete this.items[this.lowestCount]
        this.lowestCount++  // 头元素索引增加一
        return result
    }
    peek(){     // 查询队列头元素
        if(isEmpty()){
            return undefined
        }
        return this.items[this.lowestCount]
    }
    size(){     // 数组长度
        return this.count - this.lowestCount
    }
    clear(){    // 清空数组
        this.items = {}
        this.count = 0
        this.lowestCount = 0
    }
    toString(){     // toString方法
        if(isEmpty()){
            return ""
        }
        let str = `${this.items[this.lowestCount]}`
        for(let i = this.lowestCount+1; i< this.count; i++){
            str = `${str},${this.items[i]}`
        }
        return str
    }

}

 双端队列

可自由选择添加/删除头元素还是尾元素

class Deque{　　// 主要写addFront方法，其余和普通队列无异
    constructor(){
        this.count = 0
        this.lowestCount = 0
        this.items = {}
    }
    isEmpty(){
        return this.count === 0
    }
    addBack(element){　　// 加至末尾
        this.items[this.count] = element
        this.count++
    }
    addFront(element){  // 默认为不能包含负键
        if(isEmpty()){  // 若队列为空，则直接调用addBack方法
            addBack(element)
        }else if(this.lowestCount > 0){     // 若头元素索引大于0，则直接使用最小索引加即可
            this.lowestCount--
            this.items[this.lowestCount] = element
        }else{      // 头元素索引为0且队列不为空，整体向后移位，新元素加至头部
            for(let i = this.count; i > 0; i--){
                this.items[i] = this.items[i-1]
            }
            this.count++
            this.items[0] = element
        }
        
    }
}