# Definition for singly-linked list.

# class ListNode:

#     def __init__(self, x):

#         self.val = x

#         self.next = None

class Solution:

    def deleteDuplicates(self, head: ListNode) -> ListNode:

        if not head:

            return None

        result = res_pointer = ListNode(None)

        pointer = ListNode(None)

        pointer.next = head

        while pointer.next.next:

            if pointer.val != pointer.next.val != pointer.next.next.val:

                res_pointer.next = pointer.next

                res_pointer = res_pointer.next

            pointer = pointer.next

        if pointer.val != pointer.next.val:

            res_pointer.next = pointer.next

            res_pointer = res_pointer.next

        res_pointer.next = None

        return result.next