使用一个栈S来存储相邻两个节点即可

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        stack<ListNode*> s;
        if(head==nullptr || head->next == nullptr){
            return head;
        }
        ListNode * p = new ListNode();
        ListNode * cur = head;
        head = p;
        while(cur!=nullptr && cur->next !=nullptr){
            s.push(cur);
            s.push(cur->next);
            cur = cur->next->next;
            p->next = s.top();
            s.pop();
            p = p->next;
            p->next = s.top();
            s.pop();
            p = p->next;
        }
        if(cur==nullptr){
            p->next = nullptr;
        }
        else if(cur->next == nullptr){
            p->next = cur;
        }

        return head->next;


    }
};