不允许通过值来交换，在更新指针时需要小心。

ListNode *swapNodes(ListNode* head)      {          ListNode dummy(-1);          dummy.next = head;          for (ListNode *prev = &dummy, *cur = prev->next, *pnext = cur->next; pnext;              prev = cur, cur = cur->next, pnext = cur->next)          {              prev->next = pnext;              cur->next = pnext->next;              pnext->next = cur;          }      }