Linked list of virtual head nodes

package com.wuhongyu.linkList;

/**
 * 使用递归的方式添加和删除链表中的元素
 * @param <E>
 */
public class LinkList<E> {

    private class Node {
        public E e;
        public Node next;

        public Node(E e, Node next) {
            this.e = e;
            this.next = next;
        }

        public Node(E e){
            this(e, null);
        }

        public Node(){
            this(null, null);
        }

        @Override
        public String toString(){
            return e.toString();
        }
    }

    //虚拟头节点
    private Node head = new Node();
    private int size;

    int getSize(){
        return size;
    }

    boolean isEmpty(){
        return size==0;
    }

    public void add(int index,E e){
        if(index < 0 || index > size){
            throw new IllegalArgumentException("输入下标错误");
        }
        add(head,index,e);
    }
    private void add(Node node ,int index,E e){
        if(index == 0){
            /*Node newNode = new Node(e);
            newNode.next = node.next;
            node.next = newNode;*/
            node.next = new Node(e,node.next);
            return ;
        }
        add(node.next ,index--,e);
    }

    public void addFirst(E e){
        add(0,e);
    }

    public void addLast(E e){
        add(size,e);
    }

    public Node remove(int index){
        if(index < 0 || index > size){
            throw new IllegalArgumentException("输入下标错误");
        }
        return remove(index,head);
    }

    private Node remove(int index,Node node){
        if(index == 0){
            Node newNode = node.next;
            node.next = newNode.next;
            return newNode;
        }
        return remove(index--,node.next);
    }

    public Node removeFirst(){
        return remove(0);
    }

    public Node removeLast(){
        return remove(size);
    }
    @Override
    public String toString(){
        StringBuilder res = new StringBuilder();

        Node cur = head.next;
        while(cur != null){
            res.append(cur + "->");
            cur = cur.next;
        }
        res.append("NULL");

        return res.toString();
    }
}