leetcode Remove Duplicates from Sorted List

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     ListNode* deleteDuplicates(ListNode* head) {
12         if(head==NULL) return NULL;
13         ListNode* p,*q;
14         p=head;
15         q=p->next;
16         while(q!=NULL){
17             while(q->val==p->val)
18             {
19                 if(q->next!=NULL) 
20                     q=q->next;
21                 else {
22                     p->next=NULL;
23                     return head;}
24             }
25             p->next=q;
26             p=q;
27             q=p->next;
28             
29         }
30         
31         return head;
32         }
33         
34 };