During the last interview, I've had similar thing happen to me -- after we discussed whiteboard design question, the candidate asked, "how would you solve this?". This resulted in a pretty nice exchange of opinions.
In a somewhat related point, last time I was designing the coding question, I tested it on my teammmates. They all solved it, and their response was very helpful, and it allowed me to clarify some points in the problem.
But did they all solve in under the interview conditions? ( time pressure, coding infront of strangers, possiblity of not getting the job that you need etc).
I can pretty much solve all leetcodes on my own computer with noone staring at me like hungry wolf eyeing its next meal.
I usually allocate up to 20 minutes for that problem during the interview, maybe extending this to 30 minutes if they are not doing too well.
My goal was that the co-workers solve it under 10 minutes -- that 3x length factor is trying to compensate for harsher interview conditions.
As for leetcodes -- I just looked at the site, and my interview question would be rated 'easy'. In fact, I'd say that most of the 'easy' questions there are too hard for the programming interview.
In a somewhat related point, last time I was designing the coding question, I tested it on my teammmates. They all solved it, and their response was very helpful, and it allowed me to clarify some points in the problem.