def universalSearch(problem, buffer, heuristic):
    root = SearchNode(position=problem.getStartState())
    buffer.push((root, 0))  # (node, priority)
    visited_states = {str(root.position): root}

    while not buffer.isEmpty():

        curr_node = buffer.pop()[0]
        curr_pos, curr_par, curr_cost, curr_heur = curr_node.unpack()

        if problem.isGoalState(curr_pos):
            return list(reversed(curr_node.backtrack()))

        for next_position, direction, cost in problem.getSuccessors(curr_pos):

            if str(next_position) not in visited_states:
                full_cost = curr_cost + cost
                full_heur = heuristic(next_position, problem)
                new_node = SearchNode(position=next_position, parent=curr_node, transition=direction, cost=full_cost, heuristic=full_heur)
                visited_states[str(next_position)] = new_node
                buffer.push((new_node, new_node.cost + new_node.heuristic))

            else:
                # already pass
                pass