021. Merge Two Sorted Lists

 1 if (l1 == nullptr) return l2;
 2     if (l2 == nullptr) return l1;
 3     ListNode dummy(-1);
 4     ListNode* pHead = &dummy;
 5     while (l1 && l2) {
 6         if (l1->val < l2->val) {
 7             pHead->next = l1;
 8             l1 = l1->next;
 9         }
10         else {
11             pHead->next = l2;
12             l2 = l2->next;
13         }
14         pHead = pHead->next;
15     }
16     pHead->next = l1 == nullptr ? l2 : l1;
17     return dummy.next;