1class Solution {
2  public:
3    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
4      ListNode *head;
5      if (!l1) return l2;
6      if (!l2) return l1;
7
8      if (l1->val < l2->val) {
9        head = l1;
10        l1 = l1->next;
11      }
12      else {
13        head = l2;
14        l2 = l2->next;
15      }
16
17      ListNode *temp = head;
18      while (l1 && l2) {
19        if (l1->val < l2->val ) {
20          temp->next = l1;
21          l1 = l1->next;
22        } else {
23          temp->next = l2;
24          l2 = l2->next;
25        }
26        temp = temp->next;
27      }
28
29      while (l1) {
30        temp->next = l1;
31        temp = temp->next;
32        l1 = l1->next;
33      }
34
35      while (l2) {
36        temp->next = l2;
37        temp = temp->next;
38        l2 = l2->next;
39      }
40
41      return head;
42    }
43};