```
# 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 head == None:
            return None
        start = head
        prev = head.val
        a = {prev:1}
        #o = [prev]
        head = head.next
        while head != None:
            if head.val == prev:
                a[prev] += 1
            else:
                prev = head.val
                a[prev] = 1
                #o.append(prev)
            head = head.next
        b = ListNode(0)
        ans = b
        for i in a: # can use for i in o
            if a[i] == 1:
                c = ListNode(i)
                b.next = c
                b = b.next
        return ans.next
        ```