/* * queue.push(start) * step = 0 * while (queue.isNotEmpty()){ * step++ * size = queue.size * //scan current level * while (size-->0){ * node = queue.pop() * //从node开始扩展 * new_nodes = expand(node) * if (goal in new_nodes){ * return step++ * } * queue.append(new_nodes) * } * } * return NOT_FOUND * */