/**
 * Definition for singly-linked list.
 * 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) {
        int step = 0;
        auto head = new ListNode();
        auto start = head;
        while(true){
            int sum = 0;
            if (l1 != nullptr) {
                sum += l1->val;
                l1 = l1->next;
            }
            if (l2 != nullptr){
                sum += l2->val;
                l2 = l2->next;
            }
            sum += step;
            step = sum / 10;
            head->val = sum % 10;
            if (l1 != nullptr || l2 != nullptr || step) {
                head->next = new ListNode();
                head = head->next;
            } else
                break;
        }
        return start;
    }
};