// 归并排序
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* list1, ListNode* list2) {
        if (list1 == nullptr) 
            return list2;
        if (list2 == nullptr) 
            return list1;

        auto dummy = new ListNode(-1);
        auto cur = dummy;
        while(list1 && list2) {
            if (list1->val < list2->val) {
                cur->next = list1;
                list1 = list1->next;
                cur = cur->next;
            } else {
                cur->next = list2;
                list2 = list2->next;
                cur = cur->next;
            }
        }

        if (list1) {
            cur->next = list1;
        }
        if (list2) {
            cur->next = list2;
        }

        return dummy->next;
    }
};