(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.

/**
 * 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) {
        if(head== NULL || head->next == NULL) return head;
        ListNode *p = head;
        ListNode *q = NULL;
        int a,b;
        while(p->next)
        {
            q=p->next;
            a=p->val;
            b=q->val;
            if(a==b)
            {
                ListNode *tmp=q;
                q=q->next;
                p->next = q;
                free(tmp);
            }
            else
            {
                p = q;
                q = q->next;
            }
        }
        return head;
    }
};