链表的代码实现

1. 双链表的代码实现

import java.util.NoSuchElementException;

public class MyLinkedList<E> {
    // 虚拟头尾节点
    final private Node<E> head, tail;
    private int size;

    // 双链表节点
    private static class Node<E> {
        E val;
        Node<E> next;
        Node<E> prev;

        Node(E val) {
            this.val = val;
        }
    }

    // 构造函数初始化虚拟头尾节点
    public MyLinkedList() {
        this.head = new Node<>(null);
        this.tail = new Node<>(null);
        head.next = tail;
        tail.prev = head;
        this.size = 0;
    }


    // ***** 增 *****

    public void addLast(E e) {
        Node<E> x = new Node<>(e);
        Node<E> temp = tail.prev;
        // temp <-> x
        temp.next = x;
        x.prev = temp;

        x.next = tail;
        tail.prev = x;
        // temp <-> x <-> tail
        size++;
    }

    public void addFirst(E e) {
        Node<E> x = new Node<>(e);
        Node<E> temp = head.next;
        // head <-> temp
        temp.prev = x;
        x.next = temp;

        head.next = x;
        x.prev = head;
        // head <-> x <-> temp
        size++;
    }

    public void add(int index, E element) {
        checkPositionIndex(index);
        if (index == size) {
            addLast(element);
            return;
        }

        // 找到 index 对应的 Node
        Node<E> p = getNode(index);
        Node<E> temp = p.prev;
        // temp <-> p

        // 新要插入的 Node
        Node<E> x = new Node<>(element);

        p.prev = x;
        temp.next = x;

        x.prev = temp;
        x.next = p;

        // temp <-> x <-> p

        size++;
    }

    // ***** 删 *****

    public E removeFirst() {
        if (size < 1) {
            throw new NoSuchElementException();
        }
        // 虚拟节点的存在是我们不用考虑空指针的问题
        Node<E> x = head.next;
        Node<E> temp = x.next;
        // head <-> x <-> temp
        head.next = temp;
        temp.prev = head;

        x.prev = null;
        x.next = null;
        // head <-> temp

        size--;
        return x.val;
    }

    public E removeLast() {
        if (size < 1) {
            throw new NoSuchElementException();
        }
        Node<E> x = tail.prev;
        Node<E> temp = tail.prev.prev;
        // temp <-> x <-> tail

        tail.prev = temp;
        temp.next = tail;

        x.prev = null;
        x.next = null;
        // temp <-> tail

        size--;
        return x.val;
    }

    public E remove(int index) {
        checkElementIndex(index);
        // 找到 index 对应的 Node
        Node<E> x = getNode(index);
        Node<E> prev = x.prev;
        Node<E> next = x.next;
        // prev <-> x <-> next
        prev.next = next;
        next.prev = prev;

        x.prev = x.next = null;
        // prev <-> next

        size--;

        return x.val;
    }

    // ***** 查 *****

    public E get(int index) {
        checkElementIndex(index);
        // 找到 index 对应的 Node
        Node<E> p = getNode(index);

        return p.val;
    }

    public E getFirst() {
        if (size < 1) {
            throw new NoSuchElementException();
        }

        return head.next.val;
    }

    public E getLast() {
        if (size < 1) {
            throw new NoSuchElementException();
        }

        return tail.prev.val;
    }

    // ***** 改 *****

    public E set(int index, E val) {
        checkElementIndex(index);
        // 找到 index 对应的 Node
        Node<E> p = getNode(index);

        E oldVal = p.val;
        p.val = val;

        return oldVal;
    }

    // ***** 其他工具函数 *****

    public int size() {
        return size;
    }

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

    private Node<E> getNode(int index) {
        checkElementIndex(index);
        Node<E> p = head.next;
        // TODO: 可以优化，通过 index 判断从 head 还是 tail 开始遍历
        for (int i = 0; i < index; i++) {
            p = p.next;
        }
        return p;
    }

    private boolean isElementIndex(int index) {
        return index >= 0 && index < size;
    }

    private boolean isPositionIndex(int index) {
        return index >= 0 && index <= size;
    }

    // 检查 index 索引位置是否可以存在元素
    private void checkElementIndex(int index) {
        if (!isElementIndex(index))
            throw new IndexOutOfBoundsException("Index: " + index + ", Size: " + size);
    }

    // 检查 index 索引位置是否可以添加元素
    private void checkPositionIndex(int index) {
        if (!isPositionIndex(index))
            throw new IndexOutOfBoundsException("Index: " + index + ", Size: " + size);
    }

    private void display() {
        System.out.println("size = " + size);
        for (Node<E> p = head.next; p != tail; p = p.next) {
            System.out.print(p.val + " <-> ");
        }
        System.out.println("null");
        System.out.println();
    }

    public static void main(String[] args) {
        MyLinkedList<Integer> list = new MyLinkedList<>();
        list.addLast(1);
        list.addLast(2);
        list.addLast(3);
        list.addFirst(0);
        list.add(2, 100);

        list.display();
        // size = 5
        // 0 <-> 1 <-> 100 <-> 2 -> 3 -> null
    }
}

2. 单链表的代码实现

import java.util.NoSuchElementException;

public class MyLinkedList2<E> {

    private static class Node<E> {
        E val;
        Node<E> next;

        Node(E val) {
            this.val = val;
            this.next = null;
        }
    }

    private Node<E> head;
    // 实际的尾部节点引用
    private Node<E> tail;
    private int size;

    public MyLinkedList2() {
        this.head = new Node<>(null);
        this.tail = head;
        this.size = 0;
    }

    public void addFirst(E e) {
        Node<E> newNode = new Node<>(e);
        newNode.next = head.next;
        head.next = newNode;
        if (size == 0) {
            tail = newNode;
        }
        size++;
    }

    public void addLast(E e) {
        Node<E> newNode = new Node<>(e);
        tail.next = newNode;
        tail = newNode;
        size++;
    }

    public void add(int index, E element) {
        checkPositionIndex(index);

        if (index == size) {
            addLast(element);
            return;
        }

        Node<E> prev = head;
        for (int i = 0; i < index; i++) {
            prev = prev.next;
        }
        Node<E> newNode = new Node<>(element);
        newNode.next = prev.next;
        prev.next = newNode;
        size++;
    }

    public E removeFirst() {
        if (isEmpty()) {
            throw new NoSuchElementException();
        }
        Node<E> first = head.next;
        head.next = first.next;
        if (size == 1) {
            tail = head;
        }
        size--;
        return first.val;
    }

    public E removeLast() {
        if (isEmpty()) {
            throw new NoSuchElementException();
        }

        Node<E> prev = head;
        while (prev.next != tail) {
            prev = prev.next;
        }
        E val = tail.val;
        prev.next = null;
        tail = prev;
        size--;
        return val;
    }

    public E remove(int index) {
        checkElementIndex(index);

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

        Node<E> nodeToRemove = prev.next;
        prev.next = nodeToRemove.next;
        // 删除的是最后一个元素
        if (index == size - 1) {
            tail = prev;
        }
        size--;
        return nodeToRemove.val;
    }

    // ***** 查 *****

    public E getFirst() {
        if (isEmpty()) {
            throw new NoSuchElementException();
        }
        return head.next.val;
    }

    public E getLast() {
        if (isEmpty()) {
            throw new NoSuchElementException();
        }
        return getNode(size - 1).val;
    }

    public E get(int index) {
        checkElementIndex(index);
        Node<E> p = getNode(index);
        return p.val;
    }

    // ***** 改 *****

    public E set(int index, E element) {
        checkElementIndex(index);
        Node<E> p = getNode(index);

        E oldVal = p.val;
        p.val = element;

        return oldVal;
    }

    // ***** 其他工具函数 *****
    public int size() {
        return size;
    }

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

    private boolean isElementIndex(int index) {
        return index >= 0 && index < size;
    }

    private boolean isPositionIndex(int index) {
        return index >= 0 && index <= size;
    }

    // 检查 index 索引位置是否可以存在元素
    private void checkElementIndex(int index) {
        if (!isElementIndex(index))
            throw new IndexOutOfBoundsException("Index: " + index + ", Size: " + size);
    }

    // 检查 index 索引位置是否可以添加元素
    private void checkPositionIndex(int index) {
        if (!isPositionIndex(index))
            throw new IndexOutOfBoundsException("Index: " + index + ", Size: " + size);
    }

    // 返回 index 对应的 Node
    // 注意：请保证传入的 index 是合法的
    private Node<E> getNode(int index) {
        Node<E> p = head.next;
        for (int i = 0; i < index; i++) {
            p = p.next;
        }
        return p;
    }

    public static void main(String[] args) {
        MyLinkedList2<Integer> list = new MyLinkedList2<>();
        list.addFirst(1);
        list.addFirst(2);
        list.addLast(3);
        list.addLast(4);
        list.add(2, 5);

        System.out.println(list.removeFirst()); // 2
        System.out.println(list.removeLast()); // 4
        System.out.println(list.remove(1)); // 5

        System.out.println(list.getFirst()); // 1
        System.out.println(list.getLast()); // 3
        System.out.println(list.get(1)); // 3
    }
}