class Solution(object):
    def deleteDuplication(self, head):
        dummy = ListNode(0)
        dummy.next = head
        tail = dummy # 头有可能没 要用dummy

        while tail.next:  # 因为用了tail.next的value, 所以while tail.next
            nxt = tail.next
            while(nxt and nxt.val == tail.next.val):
                nxt = nxt.next

            if tail.next.next == nxt: # 刚好隔了一个节点,没重复,不用删
                tail = tail.next
            else:
                tail.next = nxt #用删

        return dummy.next