Add Two Numbers

/**
 * 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 flag = 0;
        if(l1 == NULL && l2 == NULL)return NULL;
 
        ListNode * head = new ListNode(0);
        ListNode * tmp =head;
        
        while(l1!=NULL && l2!=NULL)
        {
            int temp = l1->val + l2->val + flag;
            flag = temp/10;
            ListNode *node = new ListNode(temp%10);
            tmp->next = node;
            tmp = tmp -> next;
            l1 = l1->next;
            l2 = l2->next;
        }
        while(l1!=NULL)
        {
            int temp = l1->val + flag;
            flag = temp/10;
            ListNode *node = new ListNode(temp%10);
            tmp->next = node;
            tmp = tmp -> next;
            l1 = l1->next;
        }
        while(l2!=NULL)
        {
            int temp = l2->val + flag;
            flag = temp/10;
            ListNode *node = new ListNode(temp%10);
            tmp->next = node;
            tmp = tmp -> next;
            l2 = l2->next;
        }
        if(flag == 1)
        {
            ListNode *node = new ListNode(1);
            tmp->next = node;
        }
        head =head->next;
        return head;
        
    }
};