This question can solved quickly by record the result using the dummy-node.

my code is very clean.

```
class Solution {
public:
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
ListNode* dummy=new ListNode(-1);
ListNode* cur=dummy;
while(l1 && l2){
if(l1->val < l2->val){
cur->next=l1;
cur=l1;
l1=l1->next;
}
else{
cur->next=l2;
cur=l2;
l2=l2->next;
}
}
if(!l1) cur->next=l2;
if(!l2) cur->next=l1;
return dummy->next;
}
};
```