445. Add Two Numbers II

Example:

Input: (7 -> 2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 8 -> 0 -> 7

/**
 * 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) return l2;
     if(l2 == NULL) return l1;
     
     int flag = 0;
     ListNode* res = new ListNode(0);
     ListNode* tmp = new ListNode(0);
     res = tmp;
       
     l1 = reverse(l1);
     l2 = reverse(l2);
       
     while(l1 != NULL || l2 != NULL){
         int a = l1 == NULL?0:l1->val;
         int b = l2 == NULL?0:l2->val;
         ListNode *ss = new ListNode((a+b+flag)%10);
         flag = (a+b+flag) /10;
         tmp->next = ss;
         tmp = ss;
           
         l1 = l1 == NULL? NULL:l1->next;
         l2 = l2 == NULL? NULL:l2->next;
         
     }
    
     if(flag == 1){
         
         ListNode* ss = new ListNode(1);
         tmp->next = ss;
         
     }
        
        
       return reverse(res->next);
}

ListNode *reverse(ListNode *pHead) {
      
    ListNode * dummy = new ListNode(-1);

    ListNode * pCur = pHead;
    ListNode * pNext = nullptr;
    while (pCur) {
        pNext = pCur->next;
        pCur->next = dummy->next;
        dummy->next = pCur;
        pCur = pNext;
    }
    return dummy->next;
}
};