[Linked List]Remove Duplicates from Sorted List

/**
 * 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* pre=NULL,*p=head,*next=NULL;
        while(p){
            if(pre && pre->val == p->val){
                pre->next = p->next;
                delete(p);
                p = pre->next;
            }else{
                pre = p;
                p = p->next;
            }
        }
        return head;
    }
};