<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>双向链表</title>
  </head>
  <body>
    <script>
      function DoubleList() {
        this.head = null;
        this.tail = null;
        this.length = 0;

        function Node(data) {
          this.data = data;
          this.prev = null;
          this.next = null;
        }

        DoubleList.prototype.append = function (data) {
          var newNode = new Node(data);
          if (this.length == 0) {
            this.head = newNode;
          } else {
            var current = this.head;
            while (current.next) {
              current = current.next;
            }
            current.next = newNode;
          }
          this.length += 1;
        };

        DoubleList.prototype.backwardString = function (data) {
          //定义变量
          var current = this.head;
          var resultString = "";
          while (current) {
            resultString += current.data + " ";
            current = current.next;
          }
          return resultString;
        };

        DoubleList.prototype.forwardString = function (data) {
          //定义变量
          var current = this.tail;
          var resultString = "";
          while (current) {
            resultString += current.data + " ";
            current = current.prev;
          }
          return resultString;
        };

        DoubleList.prototype.toString = function (data) {
          //定义变量
          return this.backwardString();
        };

        DoubleList.prototype.insert = function (position, data) {
          if (position < 0 || position > this.length) return false;
          var newNode = new Node(data);
          if (this.length == 0) {
            this.head = newNode;
            this.tail = newNode;
          } else {
            if (position == 0) {
              this.head.prev = newNode;
              newNode.next = this.head;
              this.head = newNode;
            } else if (position == this.length) {
              newNode.prev = this.tail;
              this.tail.next = newNode;
              this.tail = newNode;
            } else {
              var current = this.head;
              var index = 0;
              while (index++ < position) {
                current = current.next;
              }
              newNode.next = current;
              newNode.prev = current.prev;
              current.prev.next = newNode;
              current.prev = newNode;
            }
          }
          this.length += 1;
          return true;
        };
        DoubleList.prototype.get = function (position, data) {
          if (position < 0 || position > this.length) return null;
          //获取元素
          var current = this.head;
          var index = 0;
          while (index++ < position) {
            current = current.next;
          }
          return current.data;
        };
        DoubleList.prototype.indexOf = function (data) {
          var current = this.head;
          var index = 0;
          //开始查找
          while (current) {
            if (current.data == data) {
              return index;
            }
            current = current.next;
            index += 1;
          }
          return -1;
        };
        DoubleList.prototype.indexOf = function (position, newData) {
          if (position < 0 || position > this.length) return false;
          var current = this.head;
          var index = 0;
          while (index++ < position) {
            current = current.next;
          }
          current.data = newData;
          return true;
        };
        DoubleList.prototype.removeAt = function (position) {
            if (position < 0 || position >= this.length) return null;
            var current=this.head
            if(this.length==1){
                this.head=null
                this.tail=null
            }else{
                if(position==0){
                    this.head.next.prev=null
                    this.head=this.head.next
                }else if(position=this.length){
                    current=this.tail
                    this.tail.next.prev=null
                    this.tail=this.tail.prev
                }else{
                    var index=0
                 
                    while(index++<position){
                        current=current.next
                    }
                    current.prev.next=current.next
                    current.next.prev=current.prev
                }
            }
            this.length-=1
            return current.data
        }
      }
    </script>
  </body>
</html>