you'd better ask the policeman---ask后加介词吗?

ask sb sth
问某人某事
不用加介词的

ask sb. to do sth. 让某人做某事（用加介词)
ask sb. sth. 问某人某事（不用加介词）

不要，
请比较：You'd better ask the policeman
You'd better ask the policeman for the place you want to go.