Похоже принцип в том, чтобы вычитать из данного числа 9 до тех пор, пока оно не станет < 10, полученное число и будет ответом. Почему это работает, я тоже не понял.

wow! adding "_" to s at beginning is really smart! so we don't need to check if the s.length() is odd or even. Thank you for your solotion. It's easy to understand.

Thanks for showing this. I tried charAt() and it told me it couldn't make a string with it. Now I see that just adding "" at the start allows you to build a string out of chars using charAt().

Really nice solution!!

Все гениальное - просто.

s.charAt() will returns character type data so when you concate with "", it will become String type

I'm not that good in Java but just wanna know why you have to assign "" with s.charAt(i2)+s.charAt(1+i2)?

Very cool! Thanks

That's really smart!

why new BigInteger(""+n).isProbablePrime(1); is not working

please mention it as consecutive primes.

Incorrect regular expression passed to replaceFirst.

it's fine to ask a question, but please make sure you mark any code like this with the spoiler tag, (I have added this already to your comment), otherwise everyone can see your code.