定义一个双向链表

public class DoubleLinkedList {
    // 当前链表中有效的元素个数
    private int size;
    // 头节点
    private Node head;
    // 尾节点
    private Node tail;
}

定义双向链表的节点类

/**
 * 双链表的节点类
 */
class Node {
    //前驱节点
    Node prev;
    //保存具体值
    int val;
    //后继节点
    Node next;

    public Node(int val) {
        this.val = val;
    }

    public Node(Node prev, int val, Node next) {
        this.prev = prev;
        this.val = val;
        this.next = next;
    }
}

增

头插法

/**
     * 头插法
     * @param val
     */
    public void addFirst(int val){
        Node node = new Node(null,val,head);
        if (head == null){
            tail = node;
        }else{
            head.prev = node;
        }
        head = node;
        size++;
    }

尾插法

 /**
     * 尾插法
     * @param val
     */
    public void addLast(int val){
        Node node = new Node(tail,val,null);
        if (tail == null){
            head = node;
        }else {
            tail.next = node;
        }
        tail = node;
        size++;
    }

根据索引插入

/**
     * 在index位置插入节点
     * @param val
     */
    public void addIndex(int index,int val) {
        if (index<0 || index>size) {
            System.err.println("索引非法!");
            return;
        } else if (index == 0) addFirst(val);
        else if (index == size) addLast(val);
        else {
            Node prev = findNode(index - 1);
            Node newNode = new Node(prev,val,prev.next);
            prev.next.prev = newNode;
            prev.next = newNode;
            size++;
        }
    }

测试

/**
 * @author happy
 */
public class Test {
    public static void main(String[] args) {
        DoubleLinkedList doubleLinkedList = new DoubleLinkedList();
        doubleLinkedList.addFirst(1);
        doubleLinkedList.addFirst(2);
        doubleLinkedList.addLast(3);
        doubleLinkedList.addLast(4);
        //2->1->3->4->null
        System.out.println(doubleLinkedList);
        doubleLinkedList.addIndex(0,5);
        doubleLinkedList.addIndex(2,7);
        doubleLinkedList.addIndex(6,9);
        //5->2->7->1->3->4->9->null
        System.out.println(doubleLinkedList);
    }
}

找到index位置的节点----二分法

/**
     * 找到index索引对应的节点，使用二分查找
     * @param index
     * @return index对应的节点Node
     */
    private Node findNode(int index) {
        Node ret = null;
        if(index < (size >> 1)){
            ret = head;
            for (int i = 0; i <index ; i++) {
                ret = ret.next;
            }
        }else {
            ret = tail;
            for (int i = size-1; i > index ; i--) {
                ret = ret.prev;
            }
        }
        return ret;
    }

toString()方法

public String toString(){
        String ret = "";
        Node node = head;
        while (node != null){
            ret += node.val + "->";
            node = node.next;
        }
        ret += null;
        return ret;
    }