Java Program For Reversing Alternate K Nodes In A Singly Linked List

// 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 is contributed by Mayank Jaiswal

// 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;
        }
    }

    /* Alternatively reverses the given
       linked list in groups of given 
       size k. */
    Node kAltReverse(Node head, int k) 
    {
        return _kAltReverse(head, k, true);
    }

    /* Helper function for kAltReverse().  
       It reverses k nodes of the list only 
       if the third parameter b is passed 
       as true, otherwise moves the pointer k 
       nodes ahead and recursively calls itself  */
    Node _kAltReverse(Node node,
                      int k, boolean b) 
    {
        if (node == null) 
        {
            return null;
        }

        int count = 1;
        Node prev = null;
        Node current = node;
        Node next = null;

        /* The loop serves two purposes
           1) If b is true, then it reverses 
           the k nodes 
           2) If b is false, then it moves
           the current pointer */
        while (current != null && count <= k) 
        {
            next = current.next;

            /* Reverse the nodes only 
               if b is true*/
            if (b == true) 
            {
                current.next = prev;
            }

            prev = current;
            current = next;
            count++;
        }

        /* 3) If b is true, then node is the 
           kth node. So attach the rest of 
           the list after the node. 
           4) After attaching, return the new 
           head */
        if (b == true) 
        {
            node.next = 
                 _kAltReverse(current, k, !b);
            return prev;
        } 

        /* If b is not true, then attach rest 
           of the list after prev. So attach rest 
           of the list after prev */ 
        else 
        {
            prev.next = _kAltReverse(current, k, !b);
            return node;
        }
    }

    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;
    }
  
    // Driver code
    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 is contributed by Mayank Jaiswal