ReverseEveryKElements.java

import java.util.*;

//class ListNode {
//    int value = 0;
//    ListNode next;
//
//    ListNode(int value) {
//        this.value = value;
//    }
//}

class ReverseEveryKElements {

    public static ListNode reverse(ListNode head, int k) {
        if (k <= 1 || head == null)
            return head;

        ListNode current = head, previous = null;
        while (true) {
            ListNode lastNodeOfPreviousPart = previous;
            // after reversing the LinkedList 'current' will become the last node of the sub-list
            ListNode lastNodeOfSubList = current;
            ListNode next = null; // will be used to temporarily store the next node
            // reverse 'k' nodes
            for (int i = 0; current != null && i < k; i++) {
                next = current.next;
                current.next = previous;
                previous = current;
                current = next;
            }

            // connect with the previous part
            if (lastNodeOfPreviousPart != null)
                lastNodeOfPreviousPart.next = previous; // 'previous' is now the first node of the sub-list
            else // this means we are changing the first node (head) of the LinkedList
                head = previous;

            // connect with the next part
            lastNodeOfSubList.next = current;

            if (current == null) // break, if we've reached the end of the LinkedList
                break;
            // prepare for the next sub-list
            previous = lastNodeOfSubList;
        }

        return head;
    }

    public static void main(String[] args) {
        ListNode head = new ListNode(1);
        head.next = new ListNode(2);
        head.next.next = new ListNode(3);
        head.next.next.next = new ListNode(4);
        head.next.next.next.next = new ListNode(5);
        head.next.next.next.next.next = new ListNode(6);
        head.next.next.next.next.next.next = new ListNode(7);
        head.next.next.next.next.next.next.next = new ListNode(8);

        ListNode result = ReverseEveryKElements.reverse(head, 3);
        System.out.print("Nodes of the reversed LinkedList are: ");
        while (result != null) {
            System.out.print(result.value + " ");
            result = result.next;
        }
    }
}