ReverseNodesKGroup_RSwap.cpp

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverse(ListNode* head){
        ListNode* c_head = NULL;
        while(head){
            ListNode* next = head->next;
            head->next = c_head;
            c_head = head;
            head = next;
        }
        return c_head;   
    }
    
    ListNode* reverseKGroup(ListNode* head, int k) {
        ListNode* dummy = new ListNode(0);
        dummy->next = head;
        ListNode* tail = dummy;
        ListNode* sub_head = head;
        ListNode* toNull = head;
        
        while(sub_head){
            
            int i = k-1;
            while(i>0){
                toNull = toNull->next;
                if(toNull == NULL)
                    return dummy->next;
                i--;
            }
            //Save the next head
            ListNode* temp = toNull->next;
            toNull->next = NULL;
            ListNode* new_sub_head = reverse(sub_head);
            
            tail->next = new_sub_head;
            tail = sub_head;
            tail->next = temp;
            sub_head = temp;
            toNull = temp;
            
        }
        return dummy->next;
        
    }
};