const swapPairs = head => {
  if (!head) return null;

  let newHead = head.next || head;

  let current = head;
  let last = null;

  while (current) {
    const temp = current;

    current = swapPair(last, current);
    last = temp;
  }

  return newHead;
};

/**
 * @return {*} next
 */
const swapPair = (last, node) => {
  const temp = node.next;
  const next = temp && temp.next;

  if (temp) {
    if (last) last.next = temp;
    temp.next = node;
    node.next = null;
  } else {
    if (last) last.next = node;
  }

  return next;
};