class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        // by: huahuajiang
        ListNode dummy(0);   // 最好不要new了, 不然还得释放
        ListNode* tail = &dummy;
        int sum = 0;
        while(l1 || l2 || sum)
        {
            sum += (l1 ? l1->val : 0) + (l2 ? l2->val : 0);
            l1 = l1 ? l1->next : nullptr;
            l2 = l2 ? l2->next : nullptr;
            tail->next = new ListNode(sum % 10);
            sum /= 10;
            tail = tail->next;
        }
        return dummy.next;
    }
}