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;    }};