题解 | 每K个一组反转链表

import java.util.Scanner;

// 定义链表节点类
class ListNode {
    int val;
    ListNode next;
    ListNode(int val) {
        this.val = val;
        this.next = null;
    }
}

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        // 读取输入的第一行，构建链表
        String[] values = scanner.nextLine().split(" ");
        ListNode head = buildLinkedList(values);

        // 读取输入的 k 值
        int k = scanner.nextInt();

        // 每 k 个节点一组翻转链表
        head = reverseKGroup(head, k);

        // 输出翻转后的链表
        printLinkedList(head);
    }

    // 根据字符串数组构建链表
    private static ListNode buildLinkedList(String[] values) {
        if (values == null || values.length == 0) {
            return null;
        }
        ListNode dummy = new ListNode(-1);
        ListNode current = dummy;
        for (String value : values) {
            current.next = new ListNode(Integer.parseInt(value));
            current = current.next;
        }
        return dummy.next;
    }

    // 每 k 个节点一组翻转链表
    private static ListNode reverseKGroup(ListNode head, int k) {
        if (head == null || k <= 1) {
            return head;
        }

        ListNode dummy = new ListNode(-1);
        dummy.next = head;

        int n = 0;
        ListNode tmp = head;
        while (tmp != null) {
            ++n;
            tmp = tmp.next;
        }

        // int t = n % k;
        ListNode p0 = dummy;
        // for (int i = 1; i <= t; ++i) {
        //     p0 = p0.next;
        // }
        
        // int len = n - t;
        int len = n;
        ListNode pre = null;
        ListNode cur = p0.next;
        while (len >= k) {
            len -= k;
            for (int i = 1; i <= k; ++i) {
                ListNode nxt = cur.next;
                cur.next = pre;
                pre = cur;
                cur = nxt;
            }
            ListNode nxt = p0.next;
            p0.next.next = cur;
            p0.next = pre;
            p0 = nxt;
        }

        return dummy.next;
    }

    // 打印链表
    private static void printLinkedList(ListNode head) {
        while (head != null) {
            System.out.print(head.val);
            if (head.next != null) {
                System.out.print(" ");
            }
            head = head.next;
        }
        System.out.println();
    }
}