leetcode[82]Remove Duplicates from Sorted List II

/**
 * 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 *tmp=head->next;
    ListNode *res=new ListNode(0);
    res->next=head;
    ListNode *p=res->next;
    ListNode *pre=res;
    int value=head->val;
    while(tmp!=NULL)
    {
        if(p->val!=tmp->val&&value!=tmp->val)
        {
            p->next=tmp;
            pre=p;
            p=p->next;
            value=tmp->val;
        }
        else
        {
            p=pre;
        }
        tmp=tmp->next;
    }
    p->next=NULL;
    return res->next;
    }
};