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) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
int carry = 0;
int sum;
ListNode *start = new ListNode(-1);
ListNode *cur = start;

while (l1 || l2 || carry) {
sum = 0;
sum += carry;

if (l1) {
sum += l1->val;
l1 = l1->next;
}

if (l2) {
sum += l2->val;
l2 = l2->next;
}

carry = sum / 10;
sum = sum % 10;

cur->next = new ListNode(sum);
cur = cur->next;
}

return start->next;
}
};