=_=

```
class Solution {
public:
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
if(l1 == NULL)return l2;
if(l2 == NULL)return l1;
if(l1->val > l2->val){
swap(l1, l2);
}
ListNode *p = l1, *q = l2, *tmp = NULL;
while(p->next != NULL){
if(p->next->val <= q->val){
p = p->next;
}else{
swap(p, q); // p always points to the tail of the list
// having been sorted
tmp = q->next;
q->next = p;
q = tmp;
}
}
p->next = q;
return l1;
}
};
```