2.cpp

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();
        ListNode* curr = dummy;
        int carry = 0;

        while (l1 || l2 || carry != 0) {
            int v1 = (l1 != nullptr) ? l1->val : 0;
            int v2 = (l2 != nullptr) ? l2->val : 0;
            int val = v1 + v2 + carry;

            carry = val / 10;
            val = val % 10;

            curr->next = new ListNode(val);

            l1 = (l1 != nullptr) ? l1->next : nullptr;
            l2 = (l2 != nullptr) ? l2->next : nullptr;
            curr = curr->next;
        }

        return dummy->next;
    }
};