Hey Peterlu,
We don't currently have an API to return you the solution leaderboard. There is also no good workaround that I know of to get that information currently. Can you please add this as an idea to our idea board.
The only other thing I can think of is creating an endpoint, iterating over all your users, and using the user solutions api to get the solutions counts to build out the leaderboard yourself. You can then have some process to call that endpoint once a day or so (so that it's never called on demand) to get the information. However, this will still be fairly inefficient and not scalable for bigger communities.
Hope this helps,
Yuri
