[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;
    }
};