/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
*/

class Solution {
public:
ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
    ListNode dummy(INT_MIN);
    ListNode *tail = &dummy;

    while (l1 && l2) {// while l1!= NULL && l2 != NULL)
        if (l1->val < l2->val) {
            tail->next = l1;
            l1 = l1->next;
        } else {
            tail->next = l2;
            l2 = l2->next;
        }
        tail = tail->next;
    }

    tail->next = l1 ? l1 : l2;
    return dummy.next;
}

ListNode *tail = new ListNode()

ListNode *tail = new ListNode(int)?