class ForumBuilder:
    def childern(self, parent, forums):
        for forum in forums:
            if forum.parent is None or parent.id != forum.parent.id:
                continue
            parent.childs.append(forum)
    def build(self, forums):
        list = []
        for forum in forums:
            if forum.parent is None:
                list.append(forum)
            self.childern(forum, forums)
        return list