AcWing.92.递归实现指数型枚举(Java)

//递归实现指数型枚举(因为结果有2< n)
        {
                for(int i = 1;i <= n;i ++)
            {
                if(st[i] == 1)
                    out.write(i + " ");
            }
            out.write("\n");
            return;
        }
        
        st[u] = 0;
        dfs(u + 1);
        st[u] = 2;
        
        st[u] = 1;
        dfs(u + 1);
        st[u] = 2;
    }
}