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