Remove Duplicates from Sorted List II.cpp

/**
 * 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) {
        ListNode* helper = new ListNode(-1);
        helper->next = head;
        auto prev = helper, curr = head;
        while(curr)
        {
            auto next = curr;
            while(next && next->val == curr->val)
                next = next->next;
            if(curr->next == next)
            {
                prev = prev->next;
                curr = curr->next;
            }
            else
            {
                prev->next = next;
                curr = next;
            }
        }
        return helper->next;
    }
};