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/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:

    ListNode* reverse(ListNode* start, ListNode* end){
        while(start != end){
            ListNode* tmp = start;
            start = start->next;
            tmp->next = end->next;
            end->next = tmp;
        }
        return start;
    }

    ListNode* reverseKGroup(ListNode* head, int k) {
        ListNode *dummy = new ListNode(0, head);
        ListNode *prev, *curr;
        prev = dummy; curr = prev->next;

        int i = 0;
        while(curr){
            i++;
            if(i == k){
                ListNode* last = prev->next;
            ListNode* tmp = curr->next;
                prev->next = reverse(prev->next, curr);
                i=0;
                last->next = tmp;
                curr = prev = last;
            }
            curr = curr->next;
        }
        return dummy->next;
    }
};