I can't remember already. Maybe I wanted to show how short I can solve this kata, but such short solution was not fast enough, therefore I redirected part of the tests to the faster version of the solution.
If I wrote 666 (just like a cool number), the short solution would be used too rarely, but I wanted it to be the main one, therefore I just added one more '6', so that only the most complex tests would be redirected to the FAST version.

Denis Nekrasov, I think it will be easier for you to read this in russian: =)

Я уже не помню. Наверное, я хотел показать, как коротко я могу решить эту ката, но такое короткое решение было недостаточно быстрым, поэтому часть тестов я перенаправлял на быстрое решение.
Если бы я написал 666 (просто красивое число), короткое решение использовалось бы слишком редко, а я хотел, чтобы оно было основным, поэтому я просто дописал ещё одну шестёрку, чтобы только самые сложные тесты перенаправлялись на FAST версию.

@B1ts he means "A positive integer with no leading zeros and a difference of at least two between adjacent digits is called a yuyuggg number.". For example, 24 is a yuyuggg number since the absolute value of (2-4) is 2, which is bigger or equal to 2 mentioned in description, while 34 is not. For edge cases, a number with a single digit is also a yuyuggg number.

I have modifyied the Description and please make sure you understand.

a difference of at least two adjacent numbers

Can you explain this part? Also, you really should have at least 1 simple example showing a small list of yu numbers.

Digital dynamic programming is the key algorithm of solving this problem