Leetcode - Add Two Numbers

/**
 * 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) {
        int sum = 0;
        ListNode* res = NULL;
        ListNode** node = &res;
        
        while(l1 != NULL || l2 != NULL || sum > 0) {
            if (l1 != NULL){
                sum += l1->val;
                l1 = l1->next;
            }
            if (l2 != NULL){
                sum += l2->val;
                l2 = l2->next;
            }
            
            (*node) = new ListNode(sum % 10);
            sum /= 10;
            node = &((*node)->next);
        }
        return res;
    }
};