non_repeat.py

class Node:
    def __init__(self, data, prev, next):
        self.data = data
        self.prev = prev
        self.next = next


class DoubleLinkedList:
    def __init__(self):
        self.head = None
        self.tail = None

    def insert(self, data):
        if self.head is None:
            self.head = Node(data, None, None)
            self.tail = self.head
        else:
            new_node = Node(data, self.tail, None)
            self.tail.next = new_node
            self.tail = new_node

        return self.tail

    def delete(self, node):
        if node == self.head:
            self.head = node.next
        if node == self.tail:
            self.tail = node.prev
        if node.prev is not None:
            node.prev.next = node.next
        if node.next is not None:
            node.next.prev = node.prev


def non_repeat(values):
    ll = DoubleLinkedList()
    counts = {}
    nodes = {}

    for v in values:
        if v not in counts:
            node = ll.insert(v)
            counts[v] = 1
            nodes[v] = node
        elif counts[v] == 1:
            node = nodes[v]
            ll.delete(node)
            counts[v] = 2

    return ll.head.data