# 25.reverse nodes in k group

## 刷题内容

• https://leetcode.com/problems/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:

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.


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/**
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* reverseKGroup(ListNode* head, int k) {
ListNode* current = head,*next1,*pre = nullptr;
int m = 1;
while(m <= k && current)
{
next1 = current ->next;
current ->next = pre;
pre = current;
current = next1;
++m;
}
if(m <= k)
{
{
ListNode* temp = pre ->next;
pre ->next = current;
current = pre;
pre = temp;
}