class Solution {
public:
    ListNode* deleteDuplicates(ListNode* head) {
        if(!head||!head->next) return head;
        auto pre=head;
        auto cur=pre->next;
        while(pre->next){
            if (cur){
                if(pre->val==cur->val) cur=cur->next;
                else {
                    pre->next=cur;
                    pre=pre->next;
                    cur=cur->next;
                }
            } else{
                pre->next=cur;
            }
        }
        return head;
    }
};