/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:

    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
      ListNode* root = new ListNode(0);
      ListNode* node = root;
      int carry = 0;
      while (l1 || l2)
      {
        int n1 = l1 ? l1->val : 0;
        int n2 = l2 ? l2->val : 0;
        int sum = n1 + n2 + carry;
        carry = sum / 10;

        node->next = new ListNode(sum % 10);
        node = node->next;

        if (l1) l1 = l1->next;
        if (l2) l2 = l2->next;
      }

      if (carry > 0)
      {
        node->next = new ListNode(carry);
      }

      return root->next;
    }
};