Issue resolved. Thanks.

I mean that with O(n^3) complexity, it will time out in some cases. You can try out this input I constructed:'A'*(250-1)+'X'+'B'*(250-1)+'X'*250+'A'+'Y'*250 . I'm sorry, but your solution actually times out.

This problem has n<=1000, but most solutions, including the Sensei's, actually has O(n^3) time complexity, and can be hacked by this input: match('A'*(250-1)+'X'+'B'*(250-1)+'X'*250+'A'+'Y'*250)

What on earth is this?!

Oops, clicked reply twice, just egnore this one

Thanks for telling me that, I'm new to here, hope that didn't bother you :)

'Cause it's drifting around in outer space, maybe orbiting the earth ;)

Note that this problem is about number theory, so better not waste time on DP or a greedy algorithm.

(?:RE) is a a non-capturing group that groups RE as a single regular expression. In this problem it's the same as (RE)