O(n)

# 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 head is None:
            return head

        walker = head
        d = {}

        if walker is not None:
            d[walker.val] = 1

        while walker is not None and walker.next is not None:
            if walker.next.val in d:
                walker.next = walker.next.next
            else:
                d[walker.next.val] = 1
                walker = walker.next

        return head

def main():
    s = Solution()

    l = [1,1,1]
    l = insert(l)
    disp(l)

    l = s.deleteDuplicates(l)
    disp(l)

def insert(nums):
    l = ListNode(nums[0])
    walker = l
    for index in range(1, len(nums)):
        walker.next = ListNode(nums[index])
        walker = walker.next

    return l

def disp(l):
    walker = l
    while walker is not None:
        print(walker.val, end = ' -> ')
        walker = walker.next
    print('None')

if __name__ == '__main__':
    main()