leetcode 21 list merge

 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         if(l1==NULL)return l2;
13         if(l2==NULL)return l1;
14         ListNode *l3,*head;
15         if(l1->val<l2->val) {
16             l3=l1;
17             l1=l1->next;
18         }
19         else {
20             l3=l2;
21             l2=l2->next;
22         }
23         head=l3;
24         while(l1!=NULL&&l2!=NULL) {
25             if(l1->val<=l2->val) {
26                 l3->next=l1;
27                 l1=l1->next;
28                 l3=l3->next;
29             }
30             else {
31                 l3->next=l2;
32                 l2=l2->next;
33                 l3=l3->next;
34             }
35         }
36         while(l1!=NULL) {
37             l3->next=l1;
38             l1=l1->next;
39             l3=l3->next;
40         }
41          while(l2!=NULL) {
42             l3->next=l2;
43             l2=l2->next;
44             l3=l3->next;
45         }
46         return head;
47     }
48 };