5、链表

1、链表

我们可以用虚拟头结点 dummyHead 来简化添加、删除的操作

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 final Node dummyHead;
    private int size;

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

    public int getSize() {
        return size;
    }

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

    /**
     * 添加
     */
    public void add(int index, E e) {
        if (index < 0 || index > size) throw new RuntimeException("need 0 <= index <= size");

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

    public void addFirst(E e) {
        add(0, e);
    }

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

    /**
     * 删除
     */
    public E remove(int index) {
        if (index < 0 || index >= size) throw new RuntimeException("need 0 <= index < size");

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

        Node delNode = prev.next;
        prev.next = delNode.next;
        delNode.next = null;
        size--;
        return delNode.e;
    }

    public E removeFirst() {
        return remove(0);
    }

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

    /**
     * 存在就删除
     */
    public void removeElement(E e) {
        Node prev = dummyHead;
        while (prev.next != null) {
            if (e.equals(prev.next.e)) {
                prev.next = prev.next.next;
                size--;
            }
            else prev = prev.next;
        }
    }

    /**
     * 修改
     */
    public void set(int index, E e) {
        if (index < 0 || index >= size) throw new RuntimeException("need 0 <= index < size");

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

    /**
     * 查看
     */
    public E get(int index) {
        if (index < 0 || index >= size) throw new RuntimeException("need 0 <= index < size");

        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 boolean contains(E e) {
        Node cur = dummyHead.next;
        while (cur != null) {
            if (cur.e.equals(e)) return true;
            cur = cur.next;
        }
        return false;
    }

    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder();
        for (Node cur = dummyHead.next; cur != null; cur = cur.next) {
            sb.append(cur).append("->");
        }
        sb.append("NULL");
        return sb.toString();
    }
}

2、链表栈

我们很容易基于上面的链表来实现栈，并且 push、pop、peek 的复杂度都是 O(1) 级别的，非常的酷

public class LinkedListStack<E> implements Stack<E> {
    
    private final LinkedList<E> list;

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

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

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

    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder();
        sb.append("LinkedListStack: Top [");
        sb.append(list);
        sb.append(']');
        return sb.toString();
    }
}

3、链表队列

我们使用两个变量 head 和 tail，分别指向链表的头和尾，用 tail 入队，用 head 出队

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;
    private Node tail;
    private int size;

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

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

    @Override
    public E dequeue() {
        if (isEmpty()) throw new RuntimeException("Queue is empty");
        
        Node retNode = head;
        head = retNode.next;
        retNode.next = null;
        size--;
        if (head == null) tail = null;
        return retNode.e;
    }

    @Override
    public E getFront() {
        if (isEmpty()) throw new RuntimeException("Queue is empty");
        return head.e;
    }

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

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

    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder();
        sb.append("LinkedListQueue: Front [");
        for (Node cur = head; cur != null; cur = cur.next) {
            sb.append(cur).append("->");
        }
        sb.append("NULL] Tail");
        return sb.toString();
    }
}