1 构建链表

• LinkedList.java

package linkedlist;

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, null);
        }

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

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

    private Node head;
    private int size;

    public LinkedList() {
        head = null;
        size = 0;
    }

    public int getSize() {
        return size;
    }

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

    //在链表头部添加元素
    public void addFirst(E e) {
//        Node node = new Node(e);
//        node.next = head;
//        head = node;

        head = new Node(e, head);  // 代替上边的三行
        size++;
    }

    /*
     * 在链表的 index (从 0 开始) 位置添加新的元素 e
     *
     * */
    public void add(int index, E e) {
        if (index < 0 || index > size) {
            throw new IllegalArgumentException("add failed");
        }
        if (index == 0) {
            addFirst(e);
        } else {
            Node prev = head;
            for (int i = 0; i < index - 1; i++) {
                prev = prev.next;
            }

//            Node node = new Node(e);
//            node.next = prev.next;
//            prev.next = node;
            prev.next = new Node(e, prev.next); // 代替上边3行

            size++;
        }

    }

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

}

2 为链表设立虚拟头结点

• 方便对链表头的操作
  

• LinkedList.java

package linkedlist;

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, null);
        }

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

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

    private Node dummyHead;
    private int size;

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

    public int getSize() {
        return size;
    }

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


    /*
     * 在链表的 index (从 0 开始) 位置添加新的元素 e
     *
     * */
    public void add(int index, E e) {
        if (index < 0 || index > size) {
            throw new IllegalArgumentException("add failed");
        }

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

//            Node node = new Node(e);
//            node.next = prev.next;
//            prev.next = node;
        prev.next = new Node(e, prev.next); // 代替上边3行

        size++;

    }

    //在链表头部添加元素
    public void addFirst(E e) {
        add(0, e);
    }


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

    // 获得链表第 index 个元素
    public E get(int index) {

        if (index < 0 || index >= size) {
            throw new IllegalArgumentException("add failed");
        }

        Node cur = dummyHead.next;

        for (int i = 0; i < index; i++) {
            cur = cur.next;
        }

        return cur.e;
    }

    public E getFirst() {
        return get(0);
    }

    public E getLast() {
        return get(size - 1);
    }

    public void set(int index, E e) {
        if (index < 0 || index >= size) {
            throw new IllegalArgumentException("add failed");
        }

        Node cur = dummyHead.next;

        for (int i = 0; i < index; i++) {
            cur = cur.next;
        }
        cur.e = e;

    }

    /*
     * 查找链表中是否存在 e
     * */
    public boolean contains(E e) {
        Node cur = dummyHead.next;

        while (cur != null) {
            if (cur.e.equals(e)) {
                return true;
            }
            cur = cur.next;
        }

        return false;
    }

    public E remove(int index) {

        if (index < 0 || index >= size) {
            throw new IllegalArgumentException("add failed");
        }

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

        Node retNode = prev.next;
        prev.next = retNode.next;
        retNode.next = null;
        size--;
        return retNode.e;

    }

    //删除第一个元素
    public E removeFirst() {
        return remove(0);
    }

    public E removeLast() {
        return remove(size - 1);
    }

    @Override
    public String toString() {

        StringBuilder res = new StringBuilder();

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

        return res.toString();
    }
}

• Main.java

package linkedlist;


public class Main {
    public static void main(String[] args) {

        LinkedList<Integer> linkedList = new LinkedList<>();

        for (int i = 5; i >= 0; i--) {
            linkedList.addFirst(i);
            System.out.println(linkedList);
        }

        linkedList.add(2, 666);
        System.out.println("位置2 插入666："+linkedList);

        linkedList.remove(2);
        System.out.println("位置2 移除666： "+linkedList);

        System.out.println(linkedList.getSize());

        linkedList.removeLast();
        System.out.println("删除最后一个："+linkedList);

        linkedList.removeFirst();
        System.out.println("删除第一个："+linkedList);
    }
}