为链表设置虚拟头节点---接上篇

public class LinkedList<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 = e;
            this.next = null;
        }

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

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

    private Node dummyHead;
    int size;

    public LinkedList() {
        dummyHead = new Node(null, null);
        size = 0;
    }

    //可以尝试传进来一个数组，把数组转换成链表的构造函数

    //获取链表中的元素个数
    public int getSize() {
        return size;
    }

    //返回链表是否为空
    public boolean isEmpty() {
        return size == 0;
    }

    //在链表头添加新的元素e
    public void addFirst(E e) {
        add(0,e);
    }

    //在链表的index(o-based)位置添加新的元素e
    //在链表中不是一个常用的操作，练习用:)
    public void add(int index, E e) {
        if (index < 0 || index > size)
            throw new IllegalArgumentException("Add failed.Illegal index.");

        Node prev = dummyHead;
        for (int i = 0; i < index; i++) {
            prev = prev.next;
        }
        prev.next = new Node(e, prev.next);
        size++;

    }

    //在链表末尾添加新的元素e
    public void addLast(E e) {
        add(size, e);
    }
}