Add Two Numbers

You are given two non-empty linked lists representing two non-negative integers. 
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.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example:

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

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

class Solution {
public:
    ListNode* addTwoNumbers(ListNode* a1, ListNode* a2){
        ListNode *res = new ListNode(-1);
        ListNode *cur = res;
        int carry = 0;
        while(a1 || a2){
            int n1 = a1 ? a1->val:0;    
            int n2 = a2 ? a2->val:0;    
            
            int sum = n1 + n2 + carry;
            carry = sum / 10;
            cur->next = new ListNode(sum % 10);
            cur = cur->next;
            if(a1) 
                a1 = a1->next;
            if(a2) 
                a2 = a2->next;
        }
        
        if(carry)
            cur->next = new ListNode(1);
        return res->next;
    }
};