2 - 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) {

int carry = 0;
ListNode * trail = new ListNode(0);
ListNode * head = trail;
while(l1 != NULL || l2 != NULL || carry == 1){ // carry  == 1 is in case of [5] + [5] = [0,1]
ListNode * newNode = new ListNode(0);

if(l1 != NULL){
newNode->val += l1->val;
l1 = l1->next;
}
if(l2 != NULL){
newNode->val += l2->val;
l2 = l2->next;
}
int temp = newNode->val + carry;
newNode->val = temp % 10;
carry = temp / 10; // carry for next node
trail->next = newNode;
trail = trail->next;
}
return head -> next;//the first node what we created, which is 0.
}
};