82. Remove Duplicates from Sorted List II (List)

Given a sorted linked list, delete all nodes that have duplicate numbers, leaving only distinct numbers from the original list.

For example,
Given 1->2->3->3->4->4->5, return 1->2->5.
Given 1->1->1->2->3, return 2->3.

class Solution {
public:
    ListNode *deleteDuplicates(ListNode *head) {
        if(!head) return head;
        
        ListNode *previous = NULL;
        ListNode *current = head;
        bool repeat = false;
        while(current)
        {
            while(current->next && current->val == current->next->val)
            {
                current = current->next;
                repeat = true;
            }
            if(repeat && previous!= NULL)
            {
                previous->next = current->next;
            }
            else if(!repeat)
            {
                if(!previous) head = current;
                previous = current;
            }
            
            current = current->next;
            repeat = false;
        }
        if(!previous) return previous;
        return head;
    }
};