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;        ListNode * cur = head->next;        while(cur!=NULL){            if(pre->val==cur->val){                cur = cur -> next;                free(pre->next);                pre->next=cur;            }            else{                pre = cur;                cur = cur->next;            }        }        return head;    }};