1 public class Solution {
 2     public ListNode Merge(ListNode list1, ListNode list2) {
 3             ListNode head;
 4             if (list1 == null) {
 5                 return list2;
 6             }
 7             if (list2 == null) {
 8                 return list1;
 9             }
10             if (list1.val < list2.val) {
11                 head = list1;
12                 head.next = Merge(list1.next, list2);
13             } else {
14                 head = list2;
15                 head.next = Merge(list1, list2.next);
16             }
17             return head;
18         }
19 }