```
class Solution {
public:
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
ListNode * result = 0;
ListNode ** result_ref = &result;
while (l1 && l2) {
if (l1->val < l2->val) {
*result_ref = l1;
l1 = l1->next;
} else {
*result_ref = l2;
l2 = l2->next;
}
result_ref = &(*result_ref)->next;
}
if (l1) *result_ref = l1;
else if (l2) *result_ref = l2;
return result;
}
};
```