Merge Two Sorted Lists

https://leetcode.com/problems/merge-two-sorted-lists/
 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 * temp;
13         ListNode * tail=NULL;
14         ListNode * res=NULL;
15         ListNode * head1=l1;
16         ListNode * head2=l2;
17         while(head1!=NULL&&head2!=NULL)
18         {
19             if(head1->val<head2->val)
20             {
21                 temp=head1;
22                 head1=head1->next;
23             }
24             else
25             {
26                 temp=head2;
27                 head2=head2->next;
28             }
29             if(res==NULL)
30             {
31                 res=temp;
32             }
33             if(tail!=NULL)
34                 tail->next=temp;
35             tail=temp;
36         }
37         while(head1!=NULL)
38         {
39             temp=head1;
40             head1=head1->next;
41             if(res==NULL)
42             {
43                 res=temp;
44             }
45             if(tail!=NULL)
46                 tail->next=temp;
47             tail=temp;
48         }
49         while(head2!=NULL)
50         {
51             temp=head2;
52             head2=head2->next;
53             if(res==NULL)
54             {
55                 res=temp;
56             }
57             if(tail!=NULL)
58                 tail->next=temp;
59             tail=temp;
60         }
61         return res;
62     }
63 };