Remove Duplicates from Sorted List II (C++)

/**
 * 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) return NULL;
        if(!head->next) return head;
        
        ListNode *dh = new ListNode(0);
        dh->next = head;
        
        ListNode *pre = dh;
        ListNode *last = head;
        ListNode *cur = head->next;
        
        int cn = 1;
        
        while(cur) {
            if(cur->val != last->val) {
                if(cn > 1) {
                    while(last != cur) {
                        pre->next = last->next;
                        delete last;
                        last = pre->next;
                    }
                    cn = 1;
                } else {
                    pre = last;
                    last = last->next;
                }
            } else {
                cn ++;
            }
            cur = cur->next;
        }
        
        if(cn>1) {
            while(last != cur) {
                pre->next = last->next;
                delete last;
                last = pre->next;
            }
        }
        
        cur = dh->next;
        delete dh;
        return cur;
    }
};