# Reverse Nodes in k-Group Problem

## Description

LeetCode Problem 25.

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.

You may not alter the values in the list’s nodes, only nodes themselves may be changed.

Example 1:

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Input: head = [1,2,3,4,5], k = 2
Output: [2,1,4,3,5]

Example 2:

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Input: head = [1,2,3,4,5], k = 3
Output: [3,2,1,4,5]

Example 3:

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Input: head = [1,2,3,4,5], k = 1
Output: [1,2,3,4,5]

Example 4:

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Input: head = [1], k = 1
Output: [1]

Constraints:

- The number of nodes in the list is in the range sz.
- 1 <= sz <= 5000
- 0 <= Node.val <= 1000
- 1 <= k <= sz

## Sample C++ Code

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class Solution {
public:
int length(ListNode * node){
int count=0;
while(node){
count++;
node=node->next;
}
return count;
}
ListNode* reverseKGroup(ListNode* head, int k) {
if (length(head) < k)return head;
ListNode * cur=head;
ListNode * prev=NULL, *next=NULL;
while (int i=0; i < k; i++){
next=cur->next;
cur->next=prev;
prev=cur;
cur=next;
}
head->next=reverseKGroup(cur, k);
return prev;
}
};