# Reverse Nodes in k-Group

Given a linked list, reverse the nodes of a linked list k at a time and return its modified list. k is a positive integer and is less than or equal to the length of the linked list. If the number of nodes is not a multiple of k then left-out nodes in the end should remain as it is.

Example:

Given this linked list: 1->2->3->4->5

For k = 2, you should return: 2->1->4->3->5

For k = 3, you should return: 3->2->1->4->5

Note:

Only constant extra memory is allowed.
You may not alter the values in the list's nodes, only nodes itself may be changed.

Solution in C++

```/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* reverseKGroup(ListNode* head, int k) {
vector<ListNode *> p(k);
ListNode *h=head;

int i=0;
while(i<k) {
if (!h) return head;
p[i++]=h;
h=h->next;
}

i--;
while(i>0) {
p[i]->next=p[i-1];
i--;
}
p[0]->next=(h ? reverseKGroup(h, k): NULL);

return p[k-1];
}
};```
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