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 *pre = head, *temp = head->next;
        
        while(temp!=NULL)
        {
            if(temp->val == pre->val)
            {
                pre->next =temp->next;
                temp = temp->next;
            }
            else
            {
                pre =temp;
                temp =temp->next;
            }
        }
        return head;
        
    }
};