// 解题思路：
// 虚拟头节点 + 节点指针两两交换。
class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        if (head == nullptr)
            return nullptr;

        auto dummy = new ListNode (-1);
        dummy->next = head;

        auto cur = dummy;
        auto p1 = head, p2 = p1->next;
        while(p1 != nullptr && p2 != nullptr) {
            auto next = p2->next;
            cur->next = p2;
            p2->next = p1;
            p1->next = next;

            cur = p1;
            p1 = cur->next;
            if (p1 != nullptr)
                p2 = p1->next;
        }

        return dummy->next;
    }
};