leetcode 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) {
        if (head == NULL || head -> next == NULL) return head;
        ListNode *ans = head;
        ListNode *pre = ans;
        ListNode *now = head -> next;
        while(now)
        {
            if (now -> val == pre -> val)
                now = now -> next;
            else
            {
                pre -> next = now;
                pre = now;
                now = now -> next;
            }
        }
        pre -> next = NULL; // 这个不能忘
        return ans;
    }
};