public class DuLinkList<T> {

    private class Node{
        private T data;
        private Node prev;
        private Node next;
        @SuppressWarnings("unused")
        public Node(){
            
        }
        public Node(T data , Node prev , Node next){
            this.data = data;
            this.prev = prev;
            this.next = next;
        }
    }
    
    private Node header;
    private Node tail;
    private int size;
    
    public DuLinkList(){
        header = null;
        tail = null;
    }
    
    public DuLinkList(T element){
        header = new Node(element, null , null);
        tail = header;
        size++;
    }
    
    public int length(){
        return size;
    }
    
    public T get(int index){
        return getNodeByIndex(index).data;
    }

    private Node getNodeByIndex(int index) {
        if(index < 0 || index > size){
            throw new IndexOutOfBoundsException("线性表索引越界");
        }
        if(index <= size/2){
            //从header节点开始搜索
            Node current = header;
            for(int i = 0 ; i <= size/2 && current != null ; i++ , current = current.next){
                if(i == index){
                    return current;
                }
            }
        } else {
            //从tail节点开始搜索
            Node current = tail;
            for(int i = size-1 ; i > size/2 && current != null ; i++ , current = current.prev){
                if(i == index){
                    return current;
                }
            }
        }
        return null;
    }
    
    public int locate(T element){
        Node current = header;
        for(int i = 0 ; i < size && current != null ; i++ , current = current.next){
            if(current.data.equals(element)){
                return i;
            }
        }
        return -1;
    }
    
    public void insert(T element , int index){
        if(index < 0 || index > size){
            throw new IndexOutOfBoundsException("线性表索引越界");
        }
        if(header == null){
            add(element);
        } else {
            if(index == 0){
                addAtHeader(element);
            } else {
                Node prev = getNodeByIndex(index - 1);
                Node next = prev.next;
                Node newNode = new Node(element , prev , next);
                prev.next = newNode;
                next.prev = newNode;
                size++;
            }
        }
    }

    private void addAtHeader(T element) {
        header = new Node(element , null , header);
        if(tail == null){
            tail = header;
        }
        size++;
    }

    public void add(T element) {
        if(header == null){
            header = new Node(element , null , null);
            tail = header;
        } else {
            Node newNode = new Node(element , tail , null);
            tail.next = newNode;
            tail = newNode;
        }
        size++;
    }
    
    public T delete(int index){
        if(index < 0 || index > size){
            throw new IndexOutOfBoundsException("线性表索引越界");
        }
        Node del = null;
        if(index == 0){
            del = header;
            header = header.next;
            header.prev = null;
            del.next = null;
        } else {
            Node prev = getNodeByIndex(index - 1);
            del = prev.next;
            prev.next = del.next;
            if(del.next != null){
                del.next.prev = del.prev;
                del.next = null;
            }
            del.prev = null;
        }
        size--;
        return del.data;
    }
    
    public T remove(){
        return delete(size - 1);
    }
    
    public boolean empty(){
        return size == 0;
    }
    
    public void clear(){
        header = null;
        tail = null;
        size = 0;
    }
    
    public String toString(){
        if(empty()){
            return "[]";
        } else {
            StringBuilder sb = new StringBuilder("[");
            for(Node current = header ; current != null ; current = current.next){
                sb.append(current.data.toString() + ", ");
            }
            int len = sb.length();
            return sb.delete(len-2, len).append("]").toString();
        }
    }
    
    public String reverseToString(){
        if(empty()){
            return "[]";
        } else {
            StringBuilder sb = new StringBuilder("[");
            for(Node current = tail ; current != null ; current = current.prev){
                sb.append(current.data.toString() + ", ");
            }
            int len = sb.length();
            return sb.delete(len-2, len).append("]").toString();
        }
    }
}

public class DuLinkListTest {

    public static void main(String[] args) {
        DuLinkList<String> list = new DuLinkList<String>();
        list.insert("aaaa", 0);
        list.add("bbbb");
        list.insert("cccc", 0);
        list.insert("dddd", 1);
        System.out.println(list);
        list.delete(2);
        System.out.println(list);
        System.out.println(list.reverseToString());
        System.out.println("cccc在顺序线性表中的位置：" + list.locate("cccc"));
        System.out.println("链表中索引1处的元素： " + list.get(1));
        list.remove();
        System.out.println("调用remove方法后的链表： " + list);
        list.delete(0);
        System.out.println("调用delete(0)后的链表： " + list);

    }

}

[cccc, dddd, aaaa, bbbb]
[cccc, dddd, bbbb]
[bbbb, dddd, cccc]
cccc在顺序线性表中的位置：0
链表中索引1处的元素： dddd
调用remove方法后的链表： [cccc, dddd]
调用delete(0)后的链表： [dddd]