function mySolution(k) {
  let length = 0;
  for(let n = this.head; n; n = n.next) { length++; }

  if(k < 1 || k > length) {
    return;
  }

  let node = this.head;
  for(let i = 0; i < length - k; i++) { node = node.next; }
  return node.data;
}
Do two rounds one to count and other one to return the kth value from last

O(n)

@param k
@returns {*}