[LeetCode] Add Two Numbers

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 res(0);
        int overflow = 0;
        ListNode *head = &res;
        
        while(l1 || l2)
        {
            int val1 = 0, val2 = 0;
            
            if(l1) 
            {
                val1 = l1->val;
                l1 = l1->next;
            }
            
            if(l2) 
            {
                val2 = l2->val;
                l2 = l2->next;
            }
            
            int sum = val1 + val2 + overflow;
            
            overflow = sum/10;
            ListNode *newNode = new ListNode(sum%10);
            head->next = newNode;
            head = newNode;
        }

        if(1 == overflow)
        {
            ListNode *newNode = new ListNode(1);
            head->next = newNode;
        }
        return res.next;

    }
};