LeetCode 21. Merge Two Sorted Lists

 1 class Solution {
 2 public:
 3     ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
 4         ListNode* head = new ListNode(0);
 5         ListNode* res = head;
 6         while(l1 != NULL && l2 != NULL){
 7             if(l1->val < l2->val){
 8                 head->next = l1;
 9                 head = head->next;
10                 l1 = l1->next;
11             }
12             else{
13                 head->next = l2;
14                 head = head->next;
15                 l2 = l2->next;
16             }
17         }
18         if(l1 != NULL) head->next = l1;
19         else if(l2 != NULL) head->next = l2;
20         return res->next;
21     }
22 };