Recursive solution for 2.2.8 Swap Nodes in Pairs

[Yobretaw]

ListNode* swapPairs(ListNode *head) {
  if(head == NULL || head->next == NULL)
    return head;

  ListNode *next = head->next;
  ListNode *temp = next->next;
  next->next = head;
  head->next = swapPairs(temp);

  return next;
}