https://leetcode-cn.com/problems/reverse-nodes-in-k-group/

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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* reverseKGroup(ListNode* head, int k) {
        if(k == 1) {
            return head;
        }
        ListNode * top = head;
        ListNode * ans;
        ListNode * ed = head;

        int cnt = 0;
        int flag = 0;
        while(head != nullptr) {
            cnt++;
            if(cnt == k) {
                cnt = 0;
                ListNode * tmp = head -> next;
                ListNode * tmp3 = head -> next;
                ListNode * tmp4 = top;
                while(top != head) {
                    ListNode * tmp2 = top -> next;
                    top -> next = tmp;
                    tmp = top;
                    top = tmp2;
                }
                head -> next = tmp;
                if(flag == 0) {
                    ans = head;
                    flag = 1;
                }
                else {
                    ed -> next = head;
                }
                ed = tmp4;
                top = tmp3;
                head = tmp3;
            }
            else
                head = head -> next;
        }
        return ans;
    }
};