Remove Duplicates from Sorted List

http://oj.leetcode.com/problems/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.

Solution:

//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) return NULL;
        ListNode * index =head;
        ListNode * p=head->next;
        while(p){
            if(index->val!=p->val){
                index=index->next;
                index->val=p->val;
            }
            p=p->next;
        }
        ListNode * del=index->next;
        index->next=NULL;
        while(del){
            ListNode * tmp = del->next;
            delete del;
            del=del->next;
        }
        return head;
    }
};