LeetCode: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 *head;
13             head->next=NULL;
14             ListNode *p=head;
15             while(l1!=NULL&&l2!=NULL)
16             {
17                 if(l1->val<l2->val)
18                 {
19                     p->next=l1;
20                     l1=l1->next;
21                 }
22                 else
23                 {
24                     p->next=l2;
25                     l2=l2->next;
26                 }
27                 p=p->next;
28             }
29             if(l1!=NULL)
30                 p->next=l1;
31             if(l2!=NULL)
32                 p->next=l2;
33             return head->next;
34         }
35 };