两个有序链表连接（归并排序中用到的）

刚才写了k个，顺手写个2个的，在链表的归并排序中很有用，效率非常好

 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         
15         if(l1==null&&l2==null) return null;
16         if(l1==null) return l2;
17         if(l2==null) return l1;
18         
19         ListNode head=new ListNode(-2);
20         ListNode tail=head;
21         while(l1!=null&&l2!=null)
22         {
23             if(l1.val>l2.val)
24             {
25                 tail.next=l2;
26                 tail=tail.next;
27                 l2=l2.next;
28             }
29             else
30             {
31                 tail.next=l1;
32                 tail=tail.next;
33                 l1=l1.next;
34             }
35         }
36         if(l1!=null) tail.next=l1;
37         if(l2!=null) tail.next=l2;
38         return head.next;
39             
40         }
41         
42         
43   
44 }

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