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

Given a linked list, write a function to reverse every alternate k nodes (where k is an input to the function) in an efficient way. Give the complexity of your algorithm.

Example:

Inputs:   1->2->3->4->5->6->7->8->9->NULL and k = 3
Output:   3->2->1->4->5->6->9->8->7->NULL. 

Method 1 (Process 2k nodes and recursively call for rest of the list): 
This method is basically an extension of the method discussed in this post. 

kAltReverse(struct node *head, int k)
  1)  Reverse first k nodes.
  2)  In the modified list head points to the kth node.  So change next 
       of head to (k+1)th node
  3)  Move the current pointer to skip next k nodes.
  4)  Call the kAltReverse() recursively for rest of the n - 2k nodes.
  5)  Return new head of the list.

Javascript

<script>
// JavaScript program to reverse
// alternate k nodes in a linked list
class Node
{
    constructor(d)
    {
        this.data = d;
        this.next = null;
    }
}

let head;

// Reverses alternate k nodes and returns
// the pointer to the new head node
function kAltReverse(node, k)
{
    let current = node;
    let next = null, prev = null;
    let 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;
}

function printList(node)
{
    while (node != null)
    {
        document.write(node.data + " ");
        node = node.next;
    }
}

function push(newdata)
{
    let mynode = new Node(newdata);
    mynode.next = head;
    head = mynode;
}

// Driver code

// Creating the linkedlist
for(let i = 20; i > 0; i--)
{
    push(i);
}
document.write("Given Linked List :<br>");
printList(head);
head = kAltReverse(head, 3);

document.write("<br>");
document.write("Modified Linked List :<br>");
printList(head);
// This code is contributed by rag2127
</script>

Output:

Given linked list
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Modified Linked list
3 2 1 4 5 6 9 8 7 10 11 12 15 14 13 16 17 18 20 19

Time Complexity: O(n)

Method 2 (Process k nodes and recursively call for rest of the list): 
The method 1 reverses the first k node and then moves the pointer to k nodes ahead. So method 1 uses two while loops and processes 2k nodes in one recursive call. 

This method processes only k nodes in a recursive call. It uses a third bool parameter b which decides whether to reverse the k elements or simply move the pointer.  

_kAltReverse(struct node *head, int k, bool b)
  1)  If b is true, then reverse first k nodes.
  2)  If b is false, then move the pointer k nodes ahead.
  3)  Call the kAltReverse() recursively for rest of the n - k nodes and link 
       rest of the modified list with end of first k nodes. 
  4)  Return new head of the list.

Javascript

<script>
// Javascript program to reverse
// alternate k nodes in a linked list
var head;

class Node
{
    constructor(val)
    {
        this.data = val;
        this.next = null;
    }
}

/* Alternatively reverses the given
   linked list in groups of given size k. */
function kAltReverse(head, 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 */
function _kAltReverse(node, k, b)
{
    if (node == null)
    {
        return null;
    }

    var count = 1;
    var prev = null;
    var current = node;
    var 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 the node is the kth
          node. So attach the rest of the list
          after 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;
    }
}

function printList(node)
{
    while (node != null)
    {
        document.write(node.data + " ");
        node = node.next;
    }
}

function push(newdata)
{
    var mynode = new Node(newdata);
    mynode.next = head;
    head = mynode;
}

// Creating the linkedlist
for (i = 20; i > 0; i--)
{
    push(i);
}
document.write("Given Linked List :<br/>");
printList(head);
head = kAltReverse(head, 3);
document.write("<br/>");
document.write("Modified Linked List :<br/>");
printList(head);
// This code is contributed by aashish1995
</script>

Output:

Given linked list
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Modified Linked list
3 2 1 4 5 6 9 8 7 10 11 12 15 14 13 16 17 18 20 19

Time Complexity: O(n) 

Auxiliary Space: O(n) for call stack

Please refer complete article on Reverse alternate K nodes in a Singly Linked List for more details!