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) return head;
        ListNode* pl=head;
        ListNode* pre=NULL;
        
        while(true)
        {
            if(pl==NULL || pl->next==NULL) return head;
            if(pl->next->val==pl->val)
            {
                int val=pl->val;
                while(pl!=NULL && pl->val==val) pl=pl->next;
                if(pre==NULL) head=pl;
                else pre->next=pl;
            }
            else
            {
                pre=pl;
                pl=pl->next;
            }
        }
    }
};