It is not a correct solution actually. I submitted it by accident while trying to figure out how it works :)

Yes, I mean that expected answers are affected by overflow.

I think there is overflow in some random tests. For example, test expects negative coefficients for input (54B+69)^10:
Test Failed
"...93971079496207360B^9[+15490333707103192320B^8+52781877816796062720B^7+118026143451446751360B^6+180973419958885018752B^5+192703178659923862560B^4+140703908227880915520B^3+67420622692526272020B^2+19144127431211163660B+2446194060654759801]" did not equal "...93971079496207360B^9[-2956410366606359296B^8-2558354404332592128B^7+7345679009189441664B^6-3494020778210497408B^5+8235737922828346400B^4-6870044361795497408B^3-6366353602311934444B^2+697383357501611504B+2446194060654759936]" For Input: (54B+69)^10
I use Scala by the way.