function Node (element){            this.element = element;            this.next = null;        }        function LinkedList(){            this.length = 0;            this.head = null;        }        LinkedList.prototype = {            //向表尾添加一个元素            append :function(element){                        var node = new Node(element),current;                if(this.head === null){                    this.head = node;                }else {                    current = this.head;                    while(current.next){                        current = current.next;                    }                    current.next = node;                }                length++;            },            //任意位置插入一个元素            insert :function(position,element){                        if(position >= 0 && position <= length){                    var node = new Node(element),                        current = this.head,                        previous,                        index = 0;                    if(position === 0 ){                        node.next = current;                        this.head = node;                    } else {                        while(index ++ < position){                            previous = current;                            current  = current.next;                        }                        node.next = current;                        previous.next = node;                    }                    length++;                    return true;                } else {                    return false;                }            },            //移除指定项            removeAt :function(position){                                if(position>-1 && position 
     
      = 0 && position <= this.length){                    var node = new DoublyNode(element),                        current = this.head,                        previous,                        index = 0;                    if(position === 0){        //第一个位置添加                        if(!this.head){                            this.head = node;                            this.tail = node;                        } else {                            node.next = current;                            current.prev = node;                            this.head = node;                        }                    } else if(position === this.length){        //最后一项添加                        current = this.tail;                        current.next = node;                        node.prev = current;                        this.tail = node;                    } else {                        while(index++ < position){                            previous = current;                            current = current.next;                        }                        node.prev  = previous;                        node.next = current;                        previous.next = node;                        current.prev = node;                    }                    length++;                    return true;                } else {                    return false;                }            },            removeAt :function(position){                //任意位置移除元素                if(position > -1 && position < this.length){                    var current = this.head,                        previous,                        index=0;                    if(position === 0){            //移除第一项                        this.head = current.next;                        if(this.length === 1){                            this.tail = null;                        } else {                            this.head.prev = null;                        }                    } else if(position === this.length-1){    //移除最后一项                        current = this.tail;                        this.tail = current.prev;                        this.tail.next = null;                    } else {                        while(index++ < position){                            previous = current;                            current = current.next;                        }                        previous.next = current.next;                        current.next.prev = previous;                    }                    this.length--;                    return current.element;                } else {                    return null;                }            },        };