2

/**
 * Definition for singly-linked list.
 * 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) {}
 * };
 */
class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        ListNode *dummy = new ListNode(0);
        ListNode *res = dummy;
        int carry = 0;

        while(l1 || l2 || carry){
            int sum = carry;
            if(l1) sum += l1->val, l1 = l1->next;
            if(l2) sum += l2->val, l2 = l2->next;
            res->next = new ListNode(sum%10);
            carry = sum /= 10;
            res = res->next;
        }
        return dummy->next;
    }
};