class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        ListNode* head = nullptr;
        ListNode* tail = nullptr;
        int carry = 0;
        while(l1 || l2)
        {
            int n1 = l1 ? l1->val : 0;
            int n2 = l2 ? l2->val : 0;
            int sum = n1 + n2 + carry;
            if(!head)
            {
                head = tail = new ListNode(sum%10);
            }
            else
            {
                tail->next = new ListNode(sum%10);
                tail = tail->next;
            }
            carry = sum / 10;
            if(l1)
            {
                l1 = l1->next;
            }
            if(l2)
            {
                l2 = l2->next;
            }
        }

        if(carry > 0)
        tail->next = new ListNode(carry);

        return head;
    }
};