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.

 

class Solution{
public:
    ListNode *deleteDuplicates(ListNode *head){
        ListNode *p,*q;
        if(head==NULL)
        {
            return head;
        }
        p=head;
        q=p->next;
        while(q!=NULL)
        {
            if(p->val==q->val)
            {
                p->next=q->next;
                free(q);
            }
            else
            {
                p=q;
            }
            q=p->next;
        }
        return head;
    }
};