重新写了下，代码看着清爽多了

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
12         ListNode tmp(-1);
13         ListNode *resHead = &tmp;
14         ListNode *resHead_output = resHead;
15         while(l1 != NULL && l2 != NULL){
16             if(l1->val < l2->val){
17                 resHead->next = l1;
18                 l1 = l1->next;
19             }
20             else{
21                 resHead->next = l2;
22                 l2 = l2->next;
23             }
24             resHead = resHead->next;
25         }
26         if(l1){
27             resHead->next = l1;
28         }
29         if(l2){
30             resHead->next = l2;
31         }
32         return resHead_output->next;
33     }
34 };