class Q21
{
public:
    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) {}
    };

    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2)
    {
        ListNode *preHead = new ListNode(-1);

        ListNode *prev = preHead;
        while (l1 != nullptr && l2 != nullptr)
        {
            if (l1->val < l2->val)
            {
                prev->next = l1;
                l1 = l1->next;
            }
            else
            {
                prev->next = l2;
                l2 = l2->next;
            }
            prev = prev->next;
        }

        // 合并后 l1 和 l2 最多只有一个还未被合并完，我们直接将链表末尾指向未合并完的链表即可
        prev->next = l1 == nullptr ? l2 : l1;
        std::cout << "123";
        return preHead->next;
    }
};