Add Two Numbers ---- LeetCode 002

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.

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

/**
 * 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) 
    {
        ListNode headNode(-1);
        ListNode *preNode = &headNode;
        
        int carry = 0;
        while(L1 != NULL || L2 != NULL)
        {
            const int value1 = (L1 == NULL) ? 0 : L1->val;
            const int value2 = (L2 == NULL) ? 0 : L2->val;
            L1 = (L1 == NULL) ? NULL : L1->next;
            L2 = (L2 == NULL) ? NULL : L2->next;
            
            const int value = (value1 + value2 + carry) % 10;
            carry = (value1 + value2 + carry) / 10;
            preNode->next = new ListNode(value);
            preNode = preNode->next;
        }
        if(carry == 1) 
        {
            preNode->next = new ListNode(carry);
            preNode = preNode->next;
        }
        return headNode.next;
    }
};