Solution.cpp

class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        int sum = 0;
        int carrot = 0;
        bool isHead = true;
        ListNode* rhead;
        ListNode* rbefore;

        while(l1 || l2) {
            // calculate

            sum = carrot;
            if (l1)
                sum += l1->val;
            if (l2)
                sum += l2->val;
            carrot = 0;
            
            if (sum >= 10) {
                carrot++;
                sum -= 10;
            }

            // create new node
            ListNode* ln = new ListNode(sum);

            if (isHead) {
                rhead = ln;
                rbefore = rhead;
                isHead = false;
            } else {
                rbefore->next = ln;
                rbefore = ln;
            }
            if (l1)
                l1 = (l1->next);
            if (l2)
                l2 = (l2->next);
        }
        if (carrot == 1) {
            ListNode* ln = new ListNode(1);
            if (isHead) {
                rhead = ln;
                rbefore = rhead;
                isHead = false;
            } else {
                rbefore->next = ln;
                rbefore = ln;
            }
        }
        return rhead;
    }
};