[js] 请使用 js 实现一个双向链表

链表结构是我们在面试中经常会被问起的较为基础的数据结构问题，起初学习数据结构使用的是C++语言，最近在做前端面试题的过程中没碰到了需要用js实现双链表的需求，百度出来的文章发现可很多错误，于是索性自己重新写了，并且列出了一些错误点，这里给大家较为详细的一步步看一下实现思想和步骤，相较于C++语言，js的实现可以说是很简单了，不需要创建繁琐的对象，更加直观易懂；
首先我们来看一下双向链表的结构图：
在这里插入图片描述
每个结点包含三部分，指向前一个结点的指针（pre），指向后一个节点的指针（next），以及自己的数据部分（element）,于是我们就可以先写出结点对象

function Node:定义结点对象

function Node(element) {this.element = elementthis.next = nullthis.previous = null
}

然后我们开始实现插入链表的算法
在这里插入图片描述

（声明）下面函数中的this是我们最后初始化链表的实例，这里大家不要疑惑。可以拉到最下面我们初始化链表那里，相信你会明白呦

**`function insert`:插入节点**function insert(newelement, currentelement) {var newNode = new Node(newelement)var currentNode = this.find(currentelement)if (currentNode === 'error') {console.log('无法插入，要插入节点不存在')return}if (currentNode.next != null) {newNode.next = currentNode.nextcurrentNode.next = newNodenewNode.previous = currentNodenewNode.next.previous = newNode} else {currentNode.next = newNodenewNode.previous = currentNode}
}

function find:找到插入位置

function find(element) {var currentNode = this.headwhile (currentNode.element != element) {/*如果找到最后一个节点还没有找到我们的插入点，那么我们就会返回错误*/if (currentNode.next == null) {console.log('can not find this node; maybe not have this node')return 'error'}currentNode = currentNode.next}return currentNode
}

接下来是移除结点的实现，如果看懂了插入节点的实现，那么移除就会很简单了，相信大家都可以很快明白，这里就直接贴出实现代码；

function remove:移除一个结点function remove(element) {var currentNode = this.find(element)if (currentNode === 'error') {console.log('要移除节点不存在')return}/*首先是不是头尾节点的情况*/if (currentNode.next != null && currentNode.previous != null) {currentNode.previous.next = currentNode.nextcurrentNode.next.previous = currentNode.previouscurrentNode.next = nullcurrentNode.previous = null} else if (currentNode.previous == null) {/*当是头节点的时候*/this.head = currentNode.nextcurrentNode.next.previous = nullcurrentNode.next = null} else if (currentNode.next == null) {/*当是尾节点的时候 */currentNode.previous.next = nullcurrentNode.previous = null}
}

截止到这里我们基本功能已经有了，下面使我们根据自己需要可以自定义一些其他函数

function lastNode：找到最后一个节点function lastNode() {var head = this.headwhile (head.next != null) {head = head.next}return head     //这里head在尾节点的位置
}

function append:将要添加的结点放在链表末尾function append(element) {var lastnode = this.lastNode()var newNode = new Node(element)lastnode.next = newNodenewNode.previous = lastnode
}

function showlist：将链表所有的结点打印出来function showlist() {var head = this.headdo {console.log(head.element)head = head.next} while (head != null)// 大家可以看一下下面注释内容存在什么问题，留给大家思考一下// while (head.next != null) {//   console.log(head.element)//   head = head.next// }
}

接下来是对链表进行初始化，这也是上述函数中所有this所代表的实例

function initlist:初始化链表，并将所有方法注册到链表中

function initlist() {this.head = new Node('one')this.find = findthis.insert = insertthis.remove = removethis.showlist = showlistthis.lastNode = lastNodethis.append = append
}var list = new initlist()
list.insert('two', 'one')
list.insert('four', 'two')
list.insert('three', 'two')// console.log(list.head.next)
list.showlist()
list.append('A')
list.append('B')
list.insert('B2', 'B')
list.showlist()
console.log(list.lastNode())
// list.remove('one')
// list.showlist()
console.log(list.find('A').previous)
// console.log(list.find('four').previous)
// console.log(list.head.element)

下面是运行结果：
在这里插入图片描述
源码：

function Node(element) {this.element = elementthis.next = nullthis.previous = null
}
function find(element) {var currentNode = this.headwhile (currentNode.element != element) {if (currentNode.next == null) {console.log('can not find this node; maybe not have this node')return 'error'}currentNode = currentNode.next}return currentNode
}
function insert(newelement, currentelement) {var newNode = new Node(newelement)var currentNode = this.find(currentelement)if (currentNode === 'error') {console.log('无法插入，要插入节点不存在')return}if (currentNode.next != null) {newNode.next = currentNode.nextcurrentNode.next = newNodenewNode.previous = currentNodenewNode.next.previous = newNode} else {currentNode.next = newNodenewNode.previous = currentNode}
}
function remove(element) {var currentNode = this.find(element)if (currentNode === 'error') {console.log('要移除节点不存在')return}/*首先是不是头尾节点的情况*/if (currentNode.next != null && currentNode.previous != null) {currentNode.previous.next = currentNode.nextcurrentNode.next.previous = currentNode.previouscurrentNode.next = nullcurrentNode.previous = null} else if (currentNode.previous == null) {/*当是头节点的时候*/this.head = currentNode.nextcurrentNode.next.previous = nullcurrentNode.next = null} else if (currentNode.next == null) {/*当是尾节点的时候 */currentNode.previous.next = nullcurrentNode.previous = null}
}
function showlist() {var head = this.headdo {console.log(head.element)head = head.next} while (head != null)// while (head.next != null) {//   console.log(head.element)//   head = head.next// }
}
function initlist() {this.head = new Node('one')this.find = findthis.insert = insertthis.remove = removethis.showlist = showlistthis.lastNode = lastNodethis.append = append
}
function append(element) {var lastnode = this.lastNode()var newNode = new Node(element)lastnode.next = newNodenewNode.previous = lastnode
}
function lastNode() {var head = this.headwhile (head.next != null) {head = head.next}return head
}
var list = new initlist()
list.insert('two', 'one')
list.insert('four', 'two')
list.insert('three', 'two')// console.log(list.head.next)
list.showlist()
list.append('A')
list.append('B')
list.insert('B2', 'B')
list.showlist()
console.log(list.lastNode())
// list.remove('one')
// list.showlist()
console.log(list.find('A').previous)
// console.log(list.find('four').previous)
// console.log(list.head.element)

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