// Author: Huahua
class Solution {
public:
  ListNode *swapPairs(ListNode *head) {
    if (!head || !head->next) return head;    

    ListNode d(0);
    d.next = head;
    head = &d;

    while (head && head->next && head->next->next) {
      auto n1 = head->next;
      auto n2 = n1->next;

      n1->next = n2->next;
      n2->next = n1;

      head->next = n2;
      head = n1;
    }
    return d.next;
  }
};

class Solution:
  def swapPairs(self, head):
    if not head or not head.next: return head
    dummy = ListNode(0)
    dummy.next = head
    head = dummy
    while head.next and head.next.next:
      n1, n2 = head.next, head.next.next
      n1.next = n2.next
      n2.next = n1
      head.next = n2      
      head = n1
    return dummy.next