1 class Solution {
 2     public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
 3         ListNode prehead = new ListNode(-1);
 4 
 5         ListNode prev = prehead;
 6         while (l1 != null && l2 != null) {
 7             if (l1.val <= l2.val) {
 8                 prev.next = l1;
 9                 l1 = l1.next;
10             } else {
11                 prev.next = l2;
12                 l2 = l2.next;
13             }
14             prev = prev.next;
15         }
16 
17         // 合并后 l1 和 l2 最多只有一个还未被合并完，我们直接将链表末尾指向未合并完的链表即可
18         prev.next = l1 == null ? l2 : l1;
19 
20         return prehead.next;
21     }
22 }