function DoubleLink(){
  var length=0;//鏈表長度
  var head=null;//頭結點的引用
  var tail=null;//尾節點的引用
  function Node(e){
    this.element=e;
    this.next=null;
    this.previous=null;
  }
  this.insertAt=function(position,e){//在任意位置添加節點
    if(position>=0&&position<=length){//判斷邊界
      var node=new Node(e);
      var current=head;
      var previous;
      var index=0;
      if(position==0){//在第一個位置添加
        if(!head){//鏈表為空的時候添加第一個節點
          head=node;
          tail=node;
        }else{
          current=head;
          node.next=current;
          current.previous=node;
          head=node;
        }
      }else if(position==length){//在鏈表末尾添加
        current=tail;
        current.next=node;
        node.previous=current;
        tail=node;
      }else{
        while(index<position){
          previous=current;
          current=current.next;
          index++;
        }
        previous.next=node;
        node.previous=previous;
        node.next=current;
        current.previous=node;
      }
      length++;
      return true;
    }else{
      return null;
    }
  }
  this.removeAt=function(position){//刪除任意位置的節點
    if(position>-1&&position<length){//邊界判斷
      var current=head;
      var previous;
      var index=0;
      if(position==0){//刪除第一個位置的節點
        head=current.next;
        if(length==1){//如果只有一項
          tail=null;
        }else{
          head.previous=null;
        }
      }else if(position==length-1){//刪除最后一項
        current=tail;
        tail=current.previous;
        tail.next=null;
      }else{
        while(index<position){
          previous=current;
          current=current.next;
          index++;
        }
        previous.next=current.next;
        current.next.previous=previous;
      }
      length--;
      return current.element;
    }else{
      return null;
    }
  }
  this.indexOf=function(e){//獲取節點位置，從頭開始數
    var current=head;
    var index=0;
    while(current){
      if(current.element==e){
        return index;
      }
      current=current.next;
      index++;
      if(index>=length)return null;
    }
  }
  this.isEmpty=function(){//判斷鏈表是否為空
    return length==0;
  }
  this.mylength=function(){//鏈表長度
    return length;
  }
  this.print1=function(){//從頭到尾打印鏈表
    var current=head;
    while(current){
      console.log(current.element);
      current=current.next;
    }
  }
  this.print2=function(){//從尾到頭打印鏈表
    var current=tail;
    while(current){
      console.log(current.element);
      current=current.previous;
    }
  }
  this.getHead=function(){//獲取頭節點
    return head;
  }
  this.getTail=function(){//獲取尾節點
    return tail;
  }
}
var link=new DoubleLink();//實例化一個對象
link.insertAt(0,'d');
link.insertAt(1,'e');
link.insertAt(2,'f');
link.insertAt(3,'g');
link.insertAt(4,'h');
link.insertAt(5,'i');
link.insertAt(6,'j');
link.insertAt(7,'k');
link.removeAt(7);
link.removeAt(0);
link.print1();//efghij
link.print2();//jihgfe
console.log(link.getHead());//e
console.log(link.getTail());//j
console.log(link.indexOf('f'));//1