C] 

// Java program to reverse alternate k nodes in a linked list

class LinkedList {

    static Node head;

    class Node {

        int data;
        Node next;

        Node(int d) {
            data = d;
            next = null;
        }
    }

    /* Reverses alternate k nodes and
     returns the pointer to the new head node */
    Node kAltReverse(Node node, int k) {
        Node current = node;
        Node next = null, prev = null;
        int count = 0;

        /*1) reverse first k nodes of the linked list */
        while (current != null && count < k) {
            next = current.next;
            current.next = prev;
            prev = current;
            current = next;
            count++;
        }

        /* 2) Now head points to the kth node.  So change next 
         of head to (k+1)th node*/
        if (node != null) {
            node.next = current;
        }

        /* 3) We do not want to reverse next k nodes. So move the current 
         pointer to skip next k nodes */
        count = 0;
        while (count < k - 1 && current != null) {
            current = current.next;
            count++;
        }

        /* 4) Recursively call for the list starting from current->next.
         And make rest of the list as next of first node */
        if (current != null) {
            current.next = kAltReverse(current.next, k);
        }

        /* 5) prev is new head of the input list */
        return prev;
    }

    void printList(Node node) {
        while (node != null) {
            System.out.print(node.data + " ");
            node = node.next;
        }
    }

    void push(int newdata) {
        Node mynode = new Node(newdata);
        mynode.next = head;
        head = mynode;
    }

    public static void main(String[] args) {
        LinkedList list = new LinkedList();

        // Creating the linkedlist
        for (int i = 20; i > 0; i--) {
            list.push(i);
        }
        System.out.println("Given Linked List :");
        list.printList(head);
        head = list.kAltReverse(head, 3);
        System.out.println("");
        System.out.println("Modified Linked List :");
        list.printList(head);

    }
}

// This code has been contributed by Mayank Jaiswal