Add Two Numbers

You are given two non-empty linked lists representing two non-negative integers. 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.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

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) {
        if (l1 == NULL && l2 != NULL)
                return l2;
            if (l1 != NULL && l2 == NULL)
                return l1;
            if (l1 == NULL && l2 == NULL)
                return NULL;
            ListNode *res;
            int temp = 0;
            if (l1 != NULL && l2 != NULL)
            {
                res = new ListNode((l1->val + l2->val) % 10);
                temp = (l1->val + l2->val) / 10;
                l1 = l1->next;
                l2 = l2->next;
            }
            ListNode *p = res;
            while (l1 != NULL && l2 != NULL)
            {
                ListNode *node = new ListNode((l1->val + l2->val + temp) % 10);
                temp = (l1->val + l2->val + temp) / 10;
                l1 = l1->next;
                l2 = l2->next;
                p->next = node;
                p = node;
            }
            while(l1 != NULL && l2 == NULL)
            {   
                ListNode *node = new ListNode((l1->val + temp ) % 10);
                temp = (l1->val + temp) / 10;
                l1 = l1->next;
                p->next = node;
                p = node;
            } 
            while (l2 != NULL && l1 == NULL)
            {
                ListNode *node = new ListNode((l2->val + temp) % 10);
                temp = (l2->val + temp) / 10;
                l2 = l2->next;
                p->next = node;
                p = node;
            }
            if (temp != 0)
            {
                ListNode *node = new ListNode(temp);
                p->next = node;
            }
            return res;
    }
};