(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;
}
};