24. Swap Nodes in Pairs

 1 class Solution {
 2 public:
 3     ListNode* swapPairs(ListNode* head) {
 4         if (head == nullptr || head->next == nullptr) return head;
 5         else {
 6             ListNode* curr = head; // 当前节点
 7             head = head->next; // 返回值
 8             ListNode* prev = new ListNode(-1); // 用于连接前一个翻转后的第二个节点和后一个反转后的第一个ie节点
 9             prev->next = curr;
10             ListNode* temp = prev;
11             while (curr != nullptr && curr->next != nullptr) {
12                 ListNode* first = curr;
13                 ListNode* second = curr->next;
14                 curr = second->next;
15                 first->next = curr;
16                 second->next = first;
17                 prev->next = second;
18                 prev = prev->next->next;
19             }
20             delete temp;
21             return head;
22         }
23     }
24 };