# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def deleteDuplicates(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if not head or not head.next: return head 
        dummy=ListNode(0)
        dummy.next=head
        
        pre=dummy
        start=head
        curr=head.next
        while curr:
            if curr.val==start.val:
                while curr and curr.val==start.val:
                    curr=curr.next
                pre.next=curr
                start=curr
                curr=curr.next if curr else None
            else: 
                pre,start,curr=pre.next,start.next,curr.next
        return dummy.next