```
class Solution {
public:
ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
ListNode* dummy = new ListNode(0);
ListNode* ptr = dummy;
int carry = 0;
while (l1 != NULL || l2 != NULL || carry != 0) {
int num_1 = l1 == NULL ? 0 : l1->val;
int num_2 = l2 == NULL ? 0 : l2->val;
int total = num_1 + num_2 + carry;
carry = total >= 10 ? 1 : 0;
total %= 10;
ptr->next = new ListNode(total);
ptr = ptr->next;
if (l1 != NULL) l1 = l1->next;
if (l2 != NULL) l2 = l2->next;
}
return dummy->next;
}
};
```