画图

注意有头结点

注意判断时先判断cur->next != nullptr,再判断cur->next->next != nullptr

注意末尾返回dumyhead->next，用新建result指针来接并返回

class Solution {
public:ListNode* swapPairs(ListNode* head) {ListNode *dummyhead = new ListNode(0);dummyhead->next =  head;ListNode *cur = dummyhead;while(cur->next != nullptr && cur->next->next !=nullptr){ListNode *tmp = cur->next;ListNode *tmp1 = cur->next->next->next;cur->next = cur->next->next;cur->next->next = tmp;cur->next->next->next = tmp1;cur = cur->next->next;}ListNode *result = dummyhead->next;delete dummyhead;return result;}
};