Answers for "reverse k blocks in linked list"

reverse linked list by k

//Iterative program in C++ to reverse a linked list in groups of k

//	(just before end of 2nd iteration of outer loop) [ 1->2->3->4->5->NULL ]
// 1 <- 2    3 <- 4  5 ->NULL   (k=2)
   ^		 ^		 ^
   |		 |		 |
prev_tail temp_head walker

void reverse_by_k(Node **head,int k){
    Node* temp_head = *head,*walker = *head,*prev_tail = NULL;
    while(walker){
        int i=0;
        Node *temp = NULL,*prev = NULL;
        //initialize temporary head to set previous tail later
        temp_head = walker;
        
        //reverse group of k nodes
        while(i<k && walker){
            temp = walker->next;
            walker->next = prev;
            prev = walker;
            walker = temp;
            i++;
        }
        
        if(prev_tail){
            //previous tail has to point to temporary head of current group
            prev_tail->next = prev;
            prev_tail = temp_head;
        } else{
            prev_tail = *head;
            *head = prev;
        }
    }
}