leetcode -- Merge Two Sorted Lists

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) {
 7  *         val = x;
 8  *         next = null;
 9  *     }
10  * }
11  */
12 public class Solution {
13     public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
14         // Start typing your Java solution below
15         // DO NOT write main() function
16         if(l1 == null){
17             return l2;
18         }
19         if(l2 == null){
20             return l1;
21         }
22         ListNode head = new ListNode(-1);
23         ListNode tmp = head;
24         while(l1 != null && l2 != null){
25             if(l1.val < l2.val){
26                 tmp.next = l1;
27                 l1 = l1.next;
28             } else {
29                 tmp.next = l2;
30                 l2 = l2.next;
31             }
32             tmp = tmp.next;
33         }
34         
35         if(l1 != null){
36             tmp.next = l1;
37         }
38         if(l2 != null){
39             tmp.next = l2;
40         }
41         return head.next;
42     }
43 }