Reverse Nodes in k-Group.cpp

// Contributed by: 
Name: Abhijai Rajawat
University: VIT, Vellore
Github: www.github.com/Severus25
  
// Problem Statement:
Given the head of a linked list, reverse the nodes of the list k at a time, and return the 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:
Input: head = [1,2,3,4,5], k = 2
Output: [2,1,4,3,5]

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

// Problem Solution:
class Solution {
public:
    ListNode *reverseKGroup(ListNode *head, int k) {
        if(head==NULL||k==1) return head;
        int num=0;
        ListNode *preheader = new ListNode(-1);
        preheader->next = head;
        ListNode *cur = preheader, *nex, *tmp, *pre = preheader;
        while(cur = cur->next) 
            num++;
        while(num>=k) {
            cur = pre->next;
            nex = cur->next;
            for(int i=1;i<k;i++) {
                tmp= nex->next;
                nex->next = pre->next;
                pre->next = nex;
                cur->next = tmp;
                nex = tmp;
            }
            pre = cur;
            num-=k;
        }
        return preheader->next;
    }
};