leetcode 之Swap Nodes in Pairs（21）

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(-);
          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;
          }

      }