[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 *head = new ListNode(0);
ListNode *p = head, *p1 = l1, *p2 = l2;
int val, val1, val2;
int carry = 0;
while (p1 != NULL || p2 != NULL) {
val1 = val2 = 0;
if (p1 != NULL) {
val1 = p1->val;
p1 = p1->next;
}
if (p2 != NULL) {
val2 = p2->val;
p2 = p2->next;
}
val = val1 + val2 + carry;
carry = val / 10;
val %= 10;
p->next = new ListNode(val);
p = p->next;
}
if (carry != 0) {
p->next = new ListNode(carry);
}
ListNode *sum = head->next;
delete head;
return sum;
}
};