Reverse the nodes of the linked list, k nodes at a time. If the number of nodes is not a multiple of k, the nodes left in the end will remain in their original order.

reverse-linked-list-groups.js

export function reverseLinkedList(head, k) {
  let i = 0;

  let resultHead = head;
  // node preceding our sublist
  let subPrev = null;
  // node after our sublist
  let subNext = head.next;

  // start of our sublist
  let start = head;
  // end of our sublist
  let end = head;

  while (end) {
    // if we are at the last element of a sublist
    if (i === k - 1) {
      // disconnect sublist from the rest of the list,
      // remember the next node to connect to it later
      subNext = end.next;
      end.next = null;
      if (subPrev) {
        subPrev.next = null;
      }

      // at this point our sublist is [start, end]
      // previous node is disconnected, and end is disconnected

      let prev = null;
      let curr = start;
      while (curr) {
        let next = curr.next;
        curr.next = prev;
        prev = curr;
        curr = next;
      }

      // if it's our first reversal, we need to update resultHead
      // that is going to be the head of our resulting list
      if (!subPrev) {
        resultHead = end;
      }

      // after reversal, our sublist is [end, start]
      // reconnect
      if (subPrev) {
        subPrev.next = end;
      }
      start.next = subNext;

      // at this point our sublist is reversed and reconnected back
      // prepare variables for next iteration
      i = 0;
      subPrev = start;
      start = subNext;
      end = subNext;
    } else {
      ++i;
      end = end.next;
    }
  }
  return resultHead;
}