/**
 * 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* reverseKGroup(ListNode* head, int k) {
        stack<int> my_stack;
        vector<int> v_nums;
        ListNode* tmp = head;
        int num = 0;
        while(tmp != nullptr && num < k)
        {
            num++;
            v_nums.push_back(tmp->val);
            my_stack.push(tmp->val);
            if(num == k)
            {
                while(num--) v_nums.pop_back();
                num = k;
                while(num--)
                {
                    v_nums.push_back(my_stack.top());
                    my_stack.pop();
                }
                cout<<num<<' ';
                num = 0;
            }
            tmp = tmp->next;
        }
        tmp = head;
        int i=0;
        while(tmp != nullptr)
        {
            tmp->val = v_nums[i];
            tmp = tmp->next;
            i++;
        }
        return head;
    }
};