21. Merge Two Sorted Lists Java solutions

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.

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 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) { val = x; }
 7  * }
 8  */
 9 public class Solution {
10     public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
11         if(l1 == null) return l2;
12         if(l2 == null) return l1;
13         ListNode res = new ListNode(-1);
14         ListNode pre = new ListNode(-1);
15         res.next = pre;
16         while(l1!= null && l2 != null){
17             if(l1.val <= l2.val){
18                 pre.next = l1;
19                 l1 = l1.next;
20             }else{
21                 pre.next = l2;
22                 l2 = l2.next;
23             }
24             pre = pre.next;
25         }
26         if(l1 != null) pre.next = l1;
27         if(l2 != null) pre.next = l2;
28         return res.next.next;
29     }
30 }