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 AddTwoNumbersSolution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        
        if (!l1 && !l2) return nullptr;

        ListNode* res = new ListNode(0);
        ListNode* start = res;
        ListNode* prev(nullptr);

        int carry = 0;
        while (l1 || l2) {
            res->val += (l1? l1->val : 0);
            res->val += (l2? l2->val : 0);
            res->val += carry;
            carry = res->val / 10;
            if (carry) res->val -= 10;
            
            res->next = new ListNode(0);
            prev = res;
            res = res->next;
            if (l1) l1 = l1->next;
            if (l2) l2 = l2->next;
        }
        if ((carry == 0) && prev) {
            delete prev->next;
            prev->next = nullptr;
        }
        if (carry > 0) {
            res->val++;
        }
        return start;
    }
};