```
/**
* 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 remain = 0;
int lengthL1 = 0;
int lengthL2 = 0;
ListNode* _l1 = l1;
ListNode* _l2 = l2;
// decide which one is longer
while(true){
if(_l1 != NULL){
++lengthL1;
_l1 = _l1 -> next;
}
if(_l2 != NULL){
_l2 = _l2 -> next;
++lengthL2;
}
if(_l1 == NULL && _l2 == NULL) break;
}
// always make l1 the longer one
if(lengthL1 < lengthL2){
ListNode *tmp = l1;
l1 = l2;
l2 = tmp;
}
// reset pointers
_l1 = l1;
_l2 = l2;
// in case that we need to insert at the tail
ListNode* _preL1;
while(_l1 != NULL){
if(_l2 != NULL)
_l1->val += ( _l2 -> val + remain );
else
_l1 -> val += remain;
remain = _l1 -> val / 10;
_l1 -> val = _l1 -> val % 10;
_preL1 = _l1;
_l1 = _l1 -> next;
if( _l2 != NULL)
_l2 = _l2 -> next;
}
if(remain != 0)
_preL1 -> next = new ListNode(remain);
return l1;
}
};
```