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.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* deleteDuplicates(ListNode* head) {
    ListNode *p,*q;
    p=head;
    if(head==NULL)
    return head;
    while(p->next!=NULL)
    {
        q=p;
        p=p->next;
        if(q->val==p->val)
        {
            q->next=p->next;
            p=q;

        }
    }
    return head;
    }
};