LC82. Remove Duplicates from Sorted List II

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def deleteDuplicates(self, head: Optional[ListNode]) -> Optional[ListNode]:
        """ tc O(N) sc O(1)
        """
        if not head or not head.next: return head 
        dummy  = ListNode(0,head)
        pre = dummy 
        cur = head 
        while cur:
            # if there are duplicate, keep moving cur until last point of duplicates 
            while cur.next and cur.val == cur.next.val:
                    cur = cur.next
            if pre.next == cur: # there is no duplicate between pre and cur
                pre = pre.next 
            else:
                pre.next = cur.next  # keep pre still, temtatively try pre's next to first non duplicate node 
            cur = cur.next 
        return dummy.next