代码：
// 两两交换链表的节点
class Solution {
public:
ListNode* Leecode24_swapPairs(ListNode* head) {
if (head == nullptr || head->next == nullptr)
return head;
ListNode *firstNode = head;
ListNode *secondNode = head->next;
firstNode->next = Leecode24_swapPairs(secondNode->next);
secondNode->next = firstNode;
return secondNode;
}
};
