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

// 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) {
        console.log(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);
}
console.log("Given Linked List :");
printList(head);
head = kAltReverse(head, 3);

console.log("Modified Linked List :");
printList(head);
// This code is contributed by rag2127

// Javascript program to reverse 
// alternate k nodes in a linked list
let 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;
    }

    let count = 1;
    let prev = null;
    let current = node;
    let 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) {
        console.log(node.data);
        node = node.next;
    }
}

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

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