数据结构之链表结构

链表结构是无序的，可分为单链表和双链表这两种结构。

利用JS实现单链表：

  function ListNode(item) {
      this.item = item;
      this.next = null;
    }

    function LinkedList() {
      this.head = null;
      this.length = 0;
    }

    LinkedList.prototype.append = function (item) {
      const newNode = new ListNode(item);
      if (this.length === 0) {
        this.head = newNode;
      } else {
        // 找到最后一个元素，然后添加新元素
        let current = this.head;
        while (current.next) {
          current = current.next
        }
        // 添加新元素
        current.next = newNode;
      }

      this.length += 1
    }

    LinkedList.prototype.insert = function (position, item) {
      if (position < 0) return false;
      //
      const newNode = new ListNode(item);
      if(position === 0) {
        newNode.next = this.head;
        this.head = newNode;
      } else {
        if (position > this.length) {
          this.append(item);
        } else {
          //
          let current = this.head;
          let previous = null;
          let index = 0;
          while (index < position) {
            //
            previous = current;
            current = current.next;
            //
            index++
          }
          //
          previous.next = newNode;
          newNode.next = current;
        }
      }
      
      //
      this.length += 1;
    }


    LinkedList.prototype.get = function (position) {
      if (position < 0 || position >= this.length) return '该位置对应的数据为空'
      //
      let index = 0;
      let current = this.head;
      while (index < position) {
        current = current.next;
        index++
      }
      //
      return current.item
    }

    LinkedList.prototype.removeAt = function (position) {
      if (position < 0 || position >= this.length) return '该位置对应的数据为空';
      //
      if (position === 0) {
        this.head = this.head.next
      } else {
        //
        let current = this.head;
        let previous = null;
        let index = 0;
        while (index < position) {
          //
          previous = current;
          current = current.next;
          //
          index++;
        }
        //
        previous.next = current.next;
      }
      //
      this.length -= 1;
    }

    LinkedList.prototype.toString = function (params) {
      console.log(this)
      let current = this.head;
      let result = '';
      while (current) {
        result += current.item + '-';
        //
        current = current.next
      }

      console.log(current)

      return result;
    }

　　利用JS实现双链表：

    function ListNode(item) {
      this.item = item;
      this.prev = null;
      this.next = null;
    }
    //
    function DoublyLinkedList() {
      //
      this.head = null;
      this.tail = null;
      this.length = 0;
    }
    //
    DoublyLinkedList.prototype.append = function (item) {
      const newNode = new ListNode(item);
      //
      if (this.length === 0) {
        this.head = newNode;
        this.tail = newNode;
      } else {
        newNode.prev = this.tail;
        this.tail.next = newNode;
        this.tail = newNode;
      }
      //
      this.length += 1;
    }

　　这里双链表只实现增加方法，其他的方法和单链表实现原理大同小异。相比于单链表而言，双链表增加了指向尾部变量以及每个节点增加了前指针（指向前一个元素），单链表只能单向遍历，双链表可以实现双向遍历，在某些时候可以利用二分法从后往前遍历，减少遍历时间。