Reverse_nodes_in_k_group.cpp

// Link:- https://leetcode.com/problems/reverse-nodes-in-k-group/
/**
 * 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(head==NULL || k==1) return head;
        
        ListNode *dummy = new ListNode(0);
        dummy->next=head;
        
        ListNode *curr=dummy, *prev=dummy, *nex=dummy;
        //For counting
        int count=0;
        while(curr->next!=NULL)
        {
            curr=curr->next;
            count++;
        }    
        
        // For main implementation
        while(count>=k)
        {
            curr=prev->next;
            nex=curr->next;
            for(int i=1;i<k;i++)
            {
                 curr->next=nex->next;
                 nex->next=prev->next;
                 prev->next=nex;
                 nex=curr->next;
                 
            }    
            prev=curr;
            count-=k;
        }    
        return dummy->next;
            
    }
};