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

 

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