class Solution {public:    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {        ListNode *dummy = new ListNode(-1), *cur = dummy;        while (l1 && l2) {            if (l1->val < l2->val) {                cur->next = l1;                l1 = l1->next;            } else {                cur->next = l2;                l2 = l2->next;            }            cur = cur->next;        }        cur->next = l1 ? l1 : l2;        return dummy->next;    }};