```
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
ListNode *dummy = new ListNode(0);
ListNode *cur = dummy;
while (l1 && l2) {
if (l1->val < l2->val) {
cur->next = l1;
l1 = l1->next;
} else {
cur->next = l2;
l2 = l2->next;
}
cur = cur->next;
}
if (l1)
cur->next = l1;
else
cur->next = l2;
ListNode *tmp = dummy->next;
delete dummy;
return tmp;
}
```