Don't reinvent the wheel, put them in a SQLite table, and let the sql engine create the tree for you!

With a tree you'll be limited by the RAM. I advise to use a database.

Map reduce, cluster geo-failover and CDN caching for optimized coldstarts in case you have to bring it up from scratch in a new datacenter. Bio for contact info, hourly billing. I have helped many startups reach their first 100MARR. Buy my audiobook.

Don't forget to create an index on the composite key [number, is_even]!