什么是双向链表？

双向链表

• 既可以从头遍历到尾, 又可以从尾遍历到头

• 也就是链表相连的过程是双向的. 那么它的实现原理, 你能猜到吗?

• 一个节点既有向前连接的引用, 也有一个向后连接的引用.

• 双向链表可以有效的解决单向链表中提到的问题.

双向链表的操作封装实现

        // 节点的结构   只能通过newnode调用
        class Dnode {
            constructor(data) {
                this.prev = null;
                this.data = data;
                this.next = null;
            }
        }
        // 链表的结构
        class DoubleLinkList {
            constructor() {
                this.head = null;
                this.tail = null
                this.length = 0;
            }

            // 1、向链表最后添加元素
            append(ele) {

                // 创建新节点
                let newnode = new Dnode(ele);
                // console.log(newnode);

                // 当头节点为空
                if (this.length == 0) {
                    this.head = newnode;
                    this.tail = newnode;
                } else {
                    newnode.prev = this.tail
                    this.tail.next = newnode;
                    this.tail = newnode
                }
                this.length++
            }

            // 2、向指定位置插入元素
            insert(position, ele) {
                // 位置是否合法
                if (position = 0 || position > this.length || !Number.isInteger(position)) {
                    return false
                }

                let newnode = new Dnode(ele)
                // 1、在头部插入

                if (position == 0) {
                    if (this.length == 0) {
                        this.head = newnode;
                        this.tail = newnode;
                    } else {
                        newnode.next = this.head;
                        this.head.prev = newnode
                        this.head = newnode
                    }
                    this.length++;
                } else if (position == this.length) {
                    //尾部
                    this.append(ele)
                } else {
                    let current = this.head,
                        index = 0

                    while (index < position - 1) {
                        current = current.next;
                        index++
                    }
                    newnode.prev = current;
                    newnode.next = current.next

                    current.next = newnode;
                    current.next.prev = newnode;
                    this.length++
                }

            }

            // 3、移除指定位置的元素
            removeAt(position) {
                if (position < 0 || position > this.length - 1 || !Number.isInteger(position)) {
                    return false
                }
                if (this.length == 0) {
                    return
                } else {
                    if (position == 0) {
                        if (this.length == 1) {
                            this.head = null
                            this.tail = null
                        } else {
                            this.head = this.head.next
                            this.head.prev = null
                        }
                        this.length--;
                    } else if (position == this.length - 1) {
                        this.tail = this.tail.prev
                        this.tail.next = null
                    } else {
                        let current = this.head, index = 0
                        while (index < position - 1) {
                            current = current.next;
                            index++
                        }

                        current.next = current.next.next
                        current.next.prev = current
                    }
                    this.length--
                }
            }

            // 4、查找指定元素的位置
            indexOf() {
                let current = this.head,
                    index = 0;
                while (index < this.length) {
                    if (current.data == ele) {
                        return index
                    } else {
                        //不相等时
                        current = current.next
                        index++;
                    }
                }
                return -1;
            }

            // 5、移动指定元素
            remove(ele) {
                let index = this.indexOf(ele);
                this.removeAt(index)
            }

            // 6、正向遍历
            toAfterString() {
                let current = this.head,
                    index = 0;
                while (index < this.length) {
                    res += "-" + current.data;
                    current = current.next
                    index++
                }
                return res.slice(1)
            }

            // 7、反向遍历
            toBeforeString() {
                let current = this.tail,
                    index = this.length - 1;
                while (index < this.length) {
                    res += "-" + current.data;
                    current = current.next
                    index++
                }
                return res.slice(1)
            }

        }
        let dlist = new DoubleLinkList();
        for (let i = 0; i < 5; i++) {
            list.append(i)
        }
        console.log(dlist);