LeetCode:Remove Duplicates from Sorted List

Given a sorted linked list, delete all duplicates such that each element appear only once.

For example,
Given 1->1->2, return 1->2.
Given 1->1->2->3->3, return 1->2->3.

 1 class Solution{
 2 public:
 3     ListNode *deleteDuplicates(ListNode *head){
 4         ListNode *p,*q;
 5         if(head==NULL)
 6         {
 7             return head;
 8         }
 9         p=head;
10         q=p->next;
11         while(q!=NULL)
12         {
13             if(p->val==q->val)
14             {
15                 p->next=q->next;
16                 free(q);
17             }
18             else
19             {
20                 p=q;
21             }
22             q=p->next;
23         }
24         return head;
25     }
26 };