链表

动态数组、栈、队列 底层依托于静态数组，靠 resize 解决固定容量问题
链表是真正的动态数据结构、最简单的动态数据结构
优点：不需要处理固定容量的问题
缺点：丧失了随机访问的能力（不能直接通过索引获取数据）

class Node {
    E e;
    Node next;
}

实现链表

public class LinkedList<E> {
    private int size;
    private final Node dummyHead;
    class Node {
        public E e;
        public Node next;
        Node(E e, Node next) {
            this.e = e;
            this.next = next;
        }
        Node(E e) {this(e, null); }
        Node() { this(null, null); }
        @Override
        public String toString() {
            return e.toString();
        }
    }
    public LinkedList() {
        this.dummyHead = new Node(null, null);
        size = 0;
    }
    // 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; }
    // O(n/2) = O(n)
    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 ++;
    }
    // O(1) 头部添加
    public void addFirst(E e) {
        add(0, e);
    }
    // O(n) 尾部添加
    public void addLast(E e) {
        add(size, e);
    }
    // O(n) 获得链表的第 index(0-based)个位置的元素
    // 在链表中不是一个常用操作
    public E get(int index) {
        if (index < 0 || index >= size) {
            throw new IllegalArgumentException("Get failed, Illegal index.");
        }
        Node cur = dummyHead.next;
        for(int i = 0; i < index; i++) {
            cur = cur.next;
        }
        return cur.e;
    }
    // O(n) 获得链表的最后一个元素
    public E getLast() {
        return get(size - 1);
    }
    // O(n) 修改链表的第 index(0-based)个位置的元素为 e
    public void set(int index, E e) {
        if(index < 0 || index >= size) {
            throw new IllegalArgumentException("Set failed. Illegal index.");
        }
        Node cur = dummyHead.next;
        for(int i = 0; i < index; i++) {
            cur = cur.next;
        }
        cur.e = e;
    }
    // 查找元素中是否有元素 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;
    }
    // 删除 index(0-based)位置的元素，返回删除元素
    public E remove(int index) {
        if (index < 0 || index >= size) {
            throw new IllegalArgumentException("Remove failed, Index is Illegal.");
        }
        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;
    }
    // O(1) 删除第一个元素，返回删除的元素
    public E removeFirst() {
        return remove(0);
    }
    // O(n) 删除最后一个元素，返回删除的元素
    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();
    }
}

基于链表的栈

public class LinkedListStack<E> implements Stack<E> {

    private LinkedList<E> list;

    public LinkedListStack() {
        list = new LinkedList<>();
    }

    @Override
    public int getSize() {
        return list.getSize();
    }

    @Override
    public boolean isEmpty() {
        return list.isEmpty();
    }

    @Override
    public void push(E e) {
        list.addFirst(e);
    }

    @Override
    public E pop() {
        return list.removeFirst();
    }

    @Override
    public E peek() {
        return list.getFirst();
    }

    @Override
    public String toString() {
        return "Stack: top " + list;
    }
}

基于链表的队列

public class LinkedListQueue<E> implements Queue<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, tail;
    private int size;

    public LinkedListQueue() {
        head = null;
        tail = null;
        size = 0;
    }


    @Override
    public int getSize() {
        return size;
    }

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

    @Override
    public void enqueue(E e) {
        if (tail == null) {
            tail = new Node(e);
            head = tail;
        } else  {
            tail.next = new Node(e);
            tail = tail.next;
        }
        size ++;
    }

    @Override
    public E dequeue() {
        if (isEmpty()) {
            throw new IllegalArgumentException("Cannot dequeue from an empty queue");
        }
        Node retNode = head;
        head = head.next;
        retNode.next = null;
        if (head == null) {
            tail = null;
        }
        size --;
        return retNode.e;
    }

    @Override
    public E getFront() {
        if (isEmpty()) {
            throw new IllegalArgumentException("Cannot dequeue from an empty queue");
        }
        return head.e;
    }

    @Override
    public String toString() {
        StringBuilder res = new StringBuilder();
        res.append("Queue: front ");
        Node cur = head;
        while (cur != null) {
            res.append(cur + "->");
            cur = cur.next;
        }
        res.append("NULL tail");
        return res.toString();
    }
}