（三）单链表

一、基本介绍

二、应用实例（单链表）

package linkedlist;

import java.util.Scanner;
import java.util.Stack;

public class SingleLinkedList {
    public static void main(String[] args) {
        //  以下两个链表主要用于链表合并
        LinkedList head1 = new LinkedList(0);
        LinkedList head2 = new LinkedList(0);
        insert(head1, 1);
        insert(head1, 2);
        insert(head1, 3);
        insert(head2, 4);
        insert(head2, 5);
        insert(head2, 6);

        LinkedList head = new LinkedList(0);
        Scanner scanner = new Scanner(System.in);
        boolean loop = true;
        char op;
        int pos;
        int val;
        while(loop) {
            System.out.println("\n链表插入~(i)");
            System.out.println("链表删除~(d)");
            System.out.println("链表修改~(u)");
            System.out.println("链表翻转~(r)");
            System.out.println("链表合并~(m)");
            System.out.println("链表打印~(p)");
            System.out.println("反向打印~(f)");
            System.out.println("链表插入指定位置~(c)");
            System.out.println("链表删除指定位置~(s)");
            System.out.println("链表有效节点个数~(n)");
            System.out.println("链表倒数第k个节点~(g)");
            System.out.println("退出程序(q)~");
            System.out.print("请选择你要执行的操作：");
            op = scanner.next().charAt(0);
            switch (op) {
                case 'i':
                    System.out.print("请输入要插入的值：");
                    val = scanner.nextInt();
                    insert(head, val);
                    break;
                case 'd':
                    delete(head);
                    break;
                case 'u':
                    System.out.print("请输入要更新的值：");
                    val = scanner.nextInt();
                    System.out.print("请输入元素的位置：");
                    pos =scanner.nextInt();
                    update(head, pos, val);
                    break;
                case 'r':
                    reverse(head);
                    break;
                case 'm':
                    merge(head1, head2);
                    break;
                case 'p':
                    System.out.print("单链表元素：");
                    print(head);
                    break;
                case 'f':
                    System.out.print("单链表元素(反向遍历)：");
                    reversePrint(head);
                    break;
                case 'c':
                    System.out.print("请输入要插入的值：");
                    val = scanner.nextInt();
                    System.out.print("请输入元素的位置：");
                    pos =scanner.nextInt();
                    insertByPosition(head, val, pos);
                    break;
                case 's':
                    System.out.print("请输入元素的位置：");
                    pos =scanner.nextInt();
                    deleteByPosition(head, pos);
                    break;
                case 'n':
                    count(head);
                    break;
                case 'g':
                    System.out.print("请输入k的值：");
                    pos =scanner.nextInt();
                    get(head, pos);
                    break;
                default:
                    scanner.close();
                    loop = false;
                    break;
            }
        }
    }

    public static void insert(LinkedList head, int insertVal) {
        LinkedList node = new LinkedList(insertVal);
        LinkedList temp = head;
        if (head.next == null) {
            head.next = node;
            System.out.println("结点插入成功~");
        } else {
            while (temp.next != null) {
                temp = temp.next;
            }
            temp.next = node;
            System.out.println("结点插入成功~");
        }
    }

    public static void delete(LinkedList head) {
        if (isEmpty(head)) {
            throw new RuntimeException("单链表没东西啦~");
        }
        LinkedList temp = head;
        int deleteVal = temp.next.data;
        head.next = temp.next.next;
        System.out.println("结点删除成功~");
        System.out.println("删除结点元素值：" + deleteVal);
    }

    public static void update(LinkedList head, int pos, int updateVal) {
        LinkedList temp = head;
        int count = 0;
        while (temp.next != null && count < pos) {
            count++;
            temp = temp.next;
        }
        if (temp != null) {
            temp.data = updateVal;
        } else {
            throw new RuntimeException("该元素不存在~");
        }
    }

    public static void reverse(LinkedList head) {
        if (isEmpty(head)) {
            throw new RuntimeException("链表没东西，臣妾很难做到啊~");
        }
        LinkedList pre = null;
        LinkedList curr = head.next;     // 从首结点开始翻转
        while (curr != null) {
            LinkedList next = curr.next;
            curr.next = pre;
            pre = curr;
            curr = next;
        }
        head.next = pre;
        print(head);
    }

    //  链表合并
    private static void merge(LinkedList head1, LinkedList head2) {
        if (isEmpty(head1) || isEmpty(head2)) {
            throw new RuntimeException("有个么东西啊~");
        }
        LinkedList p1 = head1.next;
        LinkedList p2 = head2.next;
        LinkedList newHead = new LinkedList(0); // 新节点的头结点
        LinkedList temp = newHead;
        while (p1 != null && p2 != null) {
            if (p1.data < p2.data) {
                temp.next = p1;
                p1 = p1.next;
            } else {
                temp.next = p2;
                p2 = p2.next;
            }
            temp = temp.next;
        }

        if (p1 != null) {
            temp.next = p1;
        } else {
            temp.next = p2;
        }

        System.out.print("合并后的链表：");
        print(newHead);
    }

    public static void print(LinkedList head) {
        if (isEmpty(head)) {
            throw new RuntimeException("单链表没东西~");
        }
        LinkedList temp = head.next;
        while (temp != null) {
            System.out.print(temp.data+ " ");
            temp = temp.next;
        }
        System.out.println();
    }

    private static void reversePrint(LinkedList head) {
        if (isEmpty(head)) {
            throw new RuntimeException("单链表没东西~");
        }
        Stack stack = new Stack<>();
        LinkedList temp = head;
        while (temp.next != null) {
            temp = temp.next;
            stack.add(temp);
        }

        while (stack.size() > 0) {
            System.out.print(stack.pop().data+" ");
        }
        System.out.println();
    }

    //  指定位置插入
    public static void insertByPosition(LinkedList head, int insertVal, int pos) {
        LinkedList temp = head;
        LinkedList node = new LinkedList(insertVal);
        int count = 0;
        while (temp.next != null && count < pos - 1) {
            count++;
            temp = temp.next;
        }
        node.next = temp.next; // temp指向的是插入位置的前一个位置
        temp.next = node;
    }

    //  指定位置删除
    public static void deleteByPosition(LinkedList head, int pos) {
        LinkedList temp = head;
        int count = 0;
        while (temp.next != null && count < pos - 1) {
            count++;
            temp = temp.next;
        }
        temp.next = temp.next.next;
    }

    private static int count(LinkedList head) {
        if (isEmpty(head)) {
            throw new RuntimeException("链表没东西，臣妾很难做到啊~");
        }
        LinkedList temp = head;
        int count = 0;
        while (temp.next != null) {
            temp = temp.next;
            count++;
        }
        System.out.println("链表有效节点个数：" + count);
        return count;
    }

    private static void get(LinkedList head, int index) {
        if (isEmpty(head)) {
            throw new RuntimeException("链表没东西，臣妾很难做到啊~");
        }
        //  1、获取链表长度
        int size = count(head);

        //  2、边界判断
        if (index <= 0 || index > size) {
            System.out.println("越界了~朋友");
        }

        //  3、遍历到size-index位置，就是倒数第k个节点
        LinkedList temp = head.next;
        for (int i = 0; i < size-index; i++) {
            temp = temp.next;
        }
        System.out.println("倒数第"+ index + "个节点的值：" + temp.data);
    }

    public static boolean isEmpty(LinkedList head) {
        if (head.next == null) {
            return true;
        }
        return false;
    }
}

class LinkedList{
    public int data;
    public LinkedList next;
    public LinkedList(int data) {
        this.data = data;
        this.next = null;
    }
}