class Node {
  constructor(value, next = null) {
    this.value = value;
    this.next = next;
  }

  toString() {
    if (this.next === null) {
      return `${this.value}`;
    }

    return `${this.value} -> ${this.next}`;
  }
}

function isEmpty(node) {
  return node === null;
}

function addKFromEndWithLength(head, k) {
  if (isEmpty(head)) {
    return [head, 0];
  }

  let first = head.value;
  let rest = head.next;

  let [restPlusK, length] = addKFromEndWithLength(rest, k);

  if ((length + 1) % k === 0) {
    return [new Node(first + k, restPlusK), length + 1]
  } else {
    return [new Node(first, restPlusK), length + 1]
  }
}

function addKFromEnd(head, k) {
  let [result, length] = addKFromEndWithLength(head, k);
  
  return result;
}

let list = [10,20,30,40,50,60,70,80,90].reduceRight((next, val) => new Node(val, next), null);

console.log('k | result');
console.log('--+----------')
for (let k = 0; k <= 9; k++) {
  let result = addKFromEnd(list, k);
  console.log("%d | %s", k, result);
}