Hi, thanks for reporting. I've changed the description an the test cases. There were multiple issues:

1. Bug in the reference solution => the test suite didn't catch the bug.
2. Words ending with -i an -u are exceptions. The correct instrumental form of szaru is szaruval and for Peti is Petivel. I added this case to exceptions and excluded them from the test cases.

Regards,

suic

Some people like the math. It is, afterall, tagged math.

substring is valid up to and including the length of the string. It would not throw on error on substring(1). In fact, it would still behave fine (empty string + 1 char).

From the String doc: Throws: IndexOutOfBoundsException - if beginIndex is negative or larger than the length of this String object.