deque.js

class Node {
    constructor() {
        this.value = null
        this.prev = null
        this.next = null
    }
}

class Deque {
    constructor() {
        this.head = null
        this.tail = null
        this.size = 0
    }

    isEmpty() {
        return this.head === null
    }

    size() {
        return this.size
    }

    addFirst(value) {
        let node = new Node()
        node.value = value
        node.next = this.head
        node.prev = null
        if (!this.head) {
            this.head = node
            this.tail = this.head
        } else {
            this.head.prev = node
            this.head = node
        }
    }

    addLast(value) {
        let node = new Node()
        node.value = value
        node.next = null
        node.prev = this.tail
        if (!this.tail) {
            this.tail = node
            this.head = this.tail
        } else {
            this.tail.next = node
            this.tail = node
        }
    }

    removeLast() {
        if (!this.tail) {
            return null
        }
        if (!this.tail.prev) {
            this.tail = null
            this.head = null
        } else {
            this.tail = this.tail.prev
            this.tail.next = null
        }
    }

    removeFirst() {
        if (!this.head) {
            return null
        }
        if (!this.head.next) {
            this.tail = null
            this.head = null
        } else {
            this.head = this.head.next
            this.head.prev = null
        }
    }

    *[Symbol.iterator]() {
        let node = this.head
        while (node) {
            yield node
            node = node.next
        }
    }
}