class Solution {
public:
    //翻转一段链表，prev一开始是这段链表的后一个listnode（不属于这段链表），然后从头依次将这段链表的listnode的指针指向prev，head是链表第一个，tail是最后一个
    pair<ListNode*, ListNode*>reverseList(ListNode *head, ListNode *tail){
        ListNode *prev = tail->next;
        ListNode *cur = head;
        while(prev != tail){
            ListNode *next = cur->next;
            cur->next = prev;
            prev = cur;
            cur = next;
        }
        return {tail, head};
    }
    ListNode* reverseKGroup(ListNode* head, int k) {//hair是不变的，prev，head，tail都在改变
        ListNode *hair = new ListNode;
        hair->next = head;
        ListNode *prev = hair, *tail = hair;
        while(head){
            for(int i = 0;i < k;i++){
                tail = tail->next;
                if(!tail){
                    return hair->next;
                }
            }
            tie(head, tail) = reverseList(head, tail);// 这里是 C++17 的写法，也可以写成
            // pair<ListNode*, ListNode*> result = myReverse(head, tail);
            // head = result.first;
            // tail = result.second;
            prev->next = head;
            head = tail->next;
            prev = tail;
        }
        return hair->next;
    }
};