Thank you. I'll have to look up Calkin–Wilf tree and Stern's Diatomic Sequence.

Please, is there a way to hide his code? No spoilers :)

(adding that they are talking about java)
and to answer the question, yes.
inital code (returning empty string) results in test output:

expected:<[]> but was:<[L)exJmASs1D?qCGTPzh.cj7XM1m/qC{Q=4gb%k<igJE.^jWQn!ux[h~R[2r@OC$SH)gbwnCu%2D?9N|Q:4>x$khdQJB%rvgShwh.vnjdG35+&6KU|1IbMmhdG3r@}@gSD2`EwnCu[2T*.Y$S97J.;m"iAJT<eC{Q0tobWo?zs1G?[*WQSz^a;mgM;I2+!6ASp!!xToJB[26=nvgSQz*ELmJB$JQ*m6gSM)VEcjqMwJ]>

Bonus issue:
s/charakters/characters

Are the expected and actual backwards?

wow! so simple, yet so complicated. thanks for the kata!

Thanks

Well done!!!

Nicely done!

Wow, even with research, this kata was challenging. Thanks kirjs!

Brute force!