function LinkedList(){
        let Node = function( element ){
            this.element = element;
            this.next = null;
        };

        let head = null;
        let length = 0;

        this.append = function( element ){
            let newNode = new Node( element );
            let current 
 
= null;
            if( head == null ){
                head = newNode;
            }else{
                current = head;
                while( current.next ) {
                    current = current.next;
                }
                current.next = newNode;
            }
            length
 
++;
        };

        this.size = function(){
            return length;
        };

        this.removeAt = function( pos ){
            if( pos > -1 && pos < length ){
                var current = head,
                    index = 0,
                    prev = null;

                
 
if( pos == 0 ){
                    head = current.next;
                }else{
                    while( index++ < pos ){
                        prev = current;
                        current = current.next;    
                    }
                    prev.next = current.next;    
                    length--;
                    return current.element;
                }
            }else{
                return null;
            }
        };

        this.print = function(){
            let current = head;
            while( current ){
                console.log( current.element );
                current = current.next;
            }
        };

        this.insert = function( pos, element ){
            let newNode = new Node( element );
            let current, prev, index = 0;
            current = head;
            if( pos >= 0 && pos <= length ){
                if( pos == 0 ){
                    newNode.next = head;
                    head = newNode;
                }else{
                    while( index++ < pos ){
                        prev = current;
                        current = current.next;    
                    }
                    newNode.next = current;    
                    prev.next = newNode;
                }
                length++;
                return true;
            }else {
                return false;
            }
        };
        this.toString = function(){
            let current = head, string = '';
            while( current ){
                string += current.element + ',';
                current = current.next;
            }
            return string.substring( 0, string.length - 1 );
        };
        this.indexOf = function( element ){
            let current = head, index = -1;
            while( current ){
                index++;
                if( current.element == element ){
                    return index;
                }
                current = current.next;
            }
            return -1;
        };
        this.remove = function( element ){
            let pos = this.indexOf( element );
            return this.removeAt( pos );
        };
        this.isEmpty = function(){
            return length == 0;
        };
        this.getHead = function(){
            return head;
        }
    }    


    var oLink = new LinkedList();
    oLink.append( "java" );
    oLink.append( "php" );
    oLink.append( "javascript" );
    oLink.append( "python" );

    oLink.print();
    console.log( "-----------------------節點個數------------------------")
    console.log( oLink.size() );

    console.log( "-----------------------刪除第2個元素之前------------------------")
    console.log( oLink.removeAt( 2 ) );
    console.log( "-----------------------刪除第2個元素之後------------------------")
    oLink.print();
    console.log( "節點個數");
    console.log( oLink.size() );
    console.log( "-----------------------插入節點前------------------------")
    oLink.insert( 0, "c語言" );
    oLink.insert( 3, "c++語言" );
    console.log( "-----------------------插入節點後------------------------")
    oLink.print();
    oLink.insert( 4, "erlang語言" );
    console.log( "-----------------------插入節點後------------------------")
    oLink.print();
    console.log( "-----------------------節點個數------------------------")
    console.log( oLink.size() );

    console.log( "-----------------------toString------------------------")
    console.log( oLink.toString() );
    console.log( "------------------------indexOf-----------------------" );
    console.log( oLink.indexOf( "c語言2" ) );
    console.log( "------------------------clear-----------------------" );
    oLink.print();