摘要：創(chuàng)建雙向鏈表創(chuàng)建節(jié)點(diǎn)繼承添加前繼指針初始化雙向鏈表對(duì)鏈表最后一個(gè)元素進(jìn)行引用插入操作尾插法任意位置添加元素刪除操作刪除特定位置的元素查詢操作查詢?cè)氐奈恢貌樵兾膊吭匦薷牟僮髑蹇针p向鏈表雙向鏈表對(duì)象轉(zhuǎn)為字符串

線性表 （List）：零個(gè)或者多個(gè)數(shù)據(jù)元素的有限序列。線性表是一個(gè)序列，具有一對(duì)一的關(guān)系，而不能具備一對(duì)多的關(guān)系。線性表要求有限性，數(shù)據(jù)類型也要相同。
本文參考的主要是大話數(shù)據(jù)結(jié)構(gòu)的原理進(jìn)行理解，用Javascript實(shí)現(xiàn)線性表的相關(guān)操作。

//創(chuàng)建節(jié)點(diǎn)
class Node{
    constructor(element){
        this.element = element;//數(shù)據(jù)域
        this.next = undefined;//指針域
    }
};
//判斷傳入的對(duì)象與鏈表的元素是否相等，待會(huì)實(shí)現(xiàn)indexOf()要用到，在這里先寫上
function(a,b){
    return a == b;
}; 
//創(chuàng)建鏈表
class LinkedList {
  constructor(equalsFn = defaultEquals) {
    this.equalsFn = equalsFn;
    this.count = 0;
    this.head = undefined;
    }
};

isEmpty() {
    return this.size() === 0;
  }

size() {
    return this.count;
  }

indexOf(element) {
    let current = this.head;
    for (let i = 0; i < this.size() && current != null; i++) {
      if (this.equalsFn(element, current.element)) {
        return i;
      }
      current = current.next;
    }
    return -1;
  }

getHead() {
    return this.head;
  }

getElementAt(index) {
    if (index >= 0 && index <= this.count) {
      let node = this.head;
      for (let i = 0; i < index && node != null; i++) {
        node = node.next;
      }
      return node;
    }
    return undefined;
  }

push(element) {
    const node = new Node(element);
    let current;
    if (this.head == null) {
      // catches null && undefined
      this.head = node;
    } else {
      current = this.head;
      while (current.next != null) {
        current = current.next;
      }
      current.next = node;
    }
    this.count++;
  }

insert(element, index) {
    if (index >= 0 && index <= this.count) {
      const node = new Node(element);
      if (index === 0) {
        const current = this.head;
        node.next = current;
        this.head = node;
      } else {
        const previous = this.getElementAt(index - 1);
        node.next = previous.next;
        previous.next = node;
      }
      this.count++;
      return true;
    }
    return false;
  }

removeAt(index) {
    if (index >= 0 && index < this.count) {
      let current = this.head;
      if (index === 0) {
        this.head = current.next;
      } else {
        const previous = this.getElementAt(index - 1);
        current = previous.next;
        previous.next = current.next;
      }
      this.count--;
      return current.element;
    }
    return undefined;
  }

remove(element) {
    const index = this.indexOf(element);
    return this.removeAt(index);
  }

toString() {
    if (this.head == null) {
      return "";
    }
    let objString = `${this.head.element}`;
    let current = this.head.next;
    for (let i = 1; i < this.size() && current != null; i++) {
      objString = `${objString},${current.element}`;
      current = current.next;
    }
    return objString;
  }
}

clear() {
    this.head = undefined;
    this.count = 0;
  }

以上就是單向鏈表的常見操作。

由于單向鏈表只能從頭到尾的遍歷，如果查詢得是下一節(jié)點(diǎn)，單向表時(shí)間復(fù)雜度為O（1），但是要查詢的是上一節(jié)點(diǎn)的話，那么時(shí)間復(fù)雜度就是O(n)。如果鏈表也可以正反遍歷的話，那么查詢操作的時(shí)間復(fù)雜度就都是O（1）啦，這時(shí)我們的前輩就提出了雙向鏈表這一神奇的鏈表。由于雙向鏈表是單向鏈表的拓展，只是多了一個(gè)指針，對(duì)于查詢操作并沒有幫助，所以實(shí)現(xiàn)方法還是跟單向鏈表一樣，這里就不多加闡述。

//創(chuàng)建節(jié)點(diǎn)
class DoublyNode extends Node {
  constructor(element, next, prev) {
    super(element, next);//繼承
    this.prev = prev;//添加前繼指針
  }
};
//初始化雙向鏈表
class DoublyLinkedList extends LinkedList {
  constructor(equalsFn = defaultEquals) {
    super(equalsFn);
    this.tail = undefined;//對(duì)鏈表最后一個(gè)元素進(jìn)行引用
  }

push(element) {
    const node = new DoublyNode(element);
    if (this.head == null) {
      this.head = node;
      this.tail = node; // NEW
    } else {
      // attach to the tail node // NEW
      this.tail.next = node;
      node.prev = this.tail;
      this.tail = node;
    }
    this.count++;
  }

insert(element, index) {
    if (index >= 0 && index <= this.count) {
      const node = new DoublyNode(element);
      let current = this.head;
      if (index === 0) {
        if (this.head == null) { // NEW
          this.head = node;
          this.tail = node; // NEW
        } else {
          node.next = this.head;
          this.head.prev = node; // NEW
          this.head = node;
        }
      } else if (index === this.count) { // last item NEW
        current = this.tail;
        current.next = node;
        node.prev = current;
        this.tail = node;
      } else {
        const previous = this.getElementAt(index - 1);
        current = previous.next;
        node.next = current;
        previous.next = node;
        current.prev = node; // NEW
        node.prev = previous; // NEW
      }
      this.count++;
      return true;
    }
    return false;
  }

removeAt(index) {
    if (index >= 0 && index < this.count) {
      let current = this.head;
      if (index === 0) {
        this.head = this.head.next;
        // if there is only one item, then we update tail as well //NEW
        if (this.count === 1) {
          // {2}
          this.tail = undefined;
        } else {
          this.head.prev = undefined;
        }
      } else if (index === this.count - 1) {
        // last item //NEW
        current = this.tail;
        this.tail = current.prev;
        this.tail.next = undefined;
      } else {
        current = this.getElementAt(index);
        const previous = current.prev;
        // link previous with current"s next - skip it to remove
        previous.next = current.next;
        current.next.prev = previous; // NEW
      }
      this.count--;
      return current.element;
    }
    return undefined;
  }

indexOf(element) {
    let current = this.head;
    let index = 0;
    while (current != null) {
      if (this.equalsFn(element, current.element)) {
        return index;
      }
      index++;
      current = current.next;
    }
    return -1;
  }

getTail() {
    return this.tail;
  }

clear() {
    super.clear();
    this.tail = undefined;
  }

inverseToString() {
    if (this.tail == null) {
      return "";
    }
    let objString = `${this.tail.element}`;
    let previous = this.tail.prev;
    while (previous != null) {
      objString = `${objString},${previous.element}`;
      previous = previous.prev;
    }
    return objString;
  }
}