代码：

/**
 * struct ListNode {
 *	int val;
 *	struct ListNode *next;
 * };
 */

/**
 * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
 * @author Senky
 * @date 2023.03.23
 * @par url https://www.nowcoder.com/creation/manager/content/584337070?type=column&status=-1
 */

 /**
 * @brief reverse a list
 * @param pHead ListNode类 
 * @return ListNode类
 */
struct ListNode* ReverseList(struct ListNode* pHead ) {
    struct ListNode* prev = NULL;
    struct ListNode* curr = pHead;
    struct ListNode* next;

    //退出循环后curr指向NULL
    while(curr != NULL)
    {
        next = curr->next;
        curr->next = prev;
        prev = curr;
        curr = next;
    }
    return prev;
}

/**
 * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
 * @param head ListNode类 
 * @param k int整型 
 * @return ListNode类
 */
struct ListNode* reverseKGroup(struct ListNode* head, int k ) {
    // write code here
    struct ListNode* reversehead = (struct ListNode*)malloc(sizeof(struct ListNode));
    struct ListNode* reversetail = (struct ListNode*)malloc(sizeof(struct ListNode));

    struct ListNode* reversehead_front = (struct ListNode*)malloc(sizeof(struct ListNode)); 
    struct ListNode* reversetail_next = (struct ListNode*)malloc(sizeof(struct ListNode));

    reversehead = head;
    reversehead_front = NULL;

    reversetail = head;
    /*reversetail_next始终指向reversetail的下一个结点*/
    reversetail_next = reversetail->next;

    /*初次翻转标记*/
    int firstreverse = 1;

    /*当前链表的尾结点的下一个不为空说明还要继续翻转*/
    while(reversetail != NULL)
    {
        /*找到反转链表的尾结点*/
        for(int reversenum = 1; reversenum < k; reversenum++)
        {
            if(reversetail->next != NULL) 
            {
                reversetail = reversetail->next;
            }
            else
            {
                return head;
            }   
        }

        if(reversetail->next != NULL) 
        {
            reversetail_next = reversetail->next;
        }
        else 
        {
            reversetail_next = NULL;
        }

        if(reversetail != NULL)
        {
            reversetail->next = NULL;
            if(1 == firstreverse) 
            {
                /*初次翻转需要改变head指向*/
                head = ReverseList(reversehead);
                firstreverse = 0;
            }
            else
            {
                /*链接翻转链表头部*/
                reversehead_front->next = ReverseList(reversehead);
            }

            reversehead_front = reversehead;//下一个循环的reversehead_front指向本次循环的reversehead
            reversehead->next = reversetail_next;//链接翻转链表尾部

            /*指针指向下一个翻转链表的头结点*/
            reversehead = reversetail_next;
            reversetail = reversetail_next;
        }
    }    
    return head;
}

图解：

注：malloc的指针记得free掉