Add two numbers represented as linked List

Add two numbers represented as linked List :

Input: (7 -> 2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 8 -> 0 -> 7

/*
 * 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) {
         if (!l1)
             return l2;
         if (!l2)
             return l1;
        int sum = 0, carry = 0;

        ListNode* head1 = reverse(l1);
        ListNode* head2 = reverse(l2);
        ListNode* result = NULL;
        ListNode* temp = NULL;   
        while (head1 || head2 || carry) {
            int v1 = !head1 ? 0 : head1->val;
            int v2 = !head2 ? 0 : head2->val;
            sum = v1 + v2;
            temp = new ListNode((sum + carry) % 10);
            temp->next = result;
            result = temp;
            carry = (sum + carry) / 10;
            head1 = !head1? NULL : head1->next;
            head2 = !head2? NULL : head2->next;
        }
        return result;
    }
   
    ListNode* reverse(ListNode* head) {
        if (!head) return NULL;
        ListNode* newHead = NULL;
        ListNode* next = NULL;
        while (head) {
            next = head->next;
            head->next = newHead;
            newHead = head;
            head = next;
        }
        return newHead;
    }
};