def search(self, start, goal):
        """Implement breadth-first search. Return the answer as a linked list of States
        where the first node contains the goal stop code and each node is linked to the previous node in the path.
        The last node in the list is the starting stop and its previous node is None.

        :param start: Code of the initial stop (str)
        :param goal: Code of the last stop (str)
        :returns (obj)
        """

        node_list = []
        visited = set()

        node_list.append(State(start))

        while len(node_list) > 0:
            node = node_list[0]
            del node_list[0]
            if node.get_stop() not in visited:
                visited.add(node.get_stop())
                if node.get_stop() == goal:
                    return node
                for neighbor in self.get_neighbors_codes(node.get_stop()):
                    node_list.append(State(neighbor,node))