Add Two Numbers

Problem: 
You are given two linked lists representing two non-negative numbers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

Solution:
/**
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int carry = 0;
        ListNode *sol = NULL;
        ListNode *temp = NULL;
   
        while (l1 != NULL || l2 != NULL) {
            int sum =  (l1 != NULL ? l1->val : 0) + (l2 != NULL ? l2->val : 0) + carry;
            carry = sum/10;
            ListNode *node = new ListNode(sum % 10);
       
            if (sol == NULL) {
                sol = node;
                temp = sol;
            }
            else {
                temp->next = node;
                temp = temp->next;
            }
            l1 = (l1 != NULL ? l1->next : NULL);
            l2 = (l2 != NULL ? l2->next : NULL);
        }
       
        if (l1 == NULL && l2 == NULL && carry) {
            ListNode *node = new ListNode(carry);
            temp->next = node;
        }
        return sol;
    }
};