What's the answer for this test?
3
1 1 1
25000000 10001 1
1 -90000 -999890001

I'm using BigInteger arithmethic and BinSearch, but still WA2...
1) D<0: answer 1
2) sum(0, k-1):f(x1+i) -> as result, we need to compare a^2*(2*k-1)^2 and 9*D

what's wrong?

Edited by author 03.04.2013 18:40

Output of my (AC) program:
1
2
190523

I used standard C++ double type to solve it - it seems there're no difficult tests that fail it (or such tests are just impossible)

BTW, for the case (2) my solution considers two different cases

I found bug in logic, but still...
1) D<0: 1
2) a) sum(0, k-1):f(x0+0.5+i) -> as result, we need to compare F(x0+0.5) and a*(k-1)/2.0 + a*(k-1)*(2*k-1)/6.0 -> then multiply by 2
   b) sum(0, k-1):f(x0+i) -> as result, we need to compare F(x0)/2.0 and a*(k-1)*(2*k-1)/6.0 -> then multiply by 2 and substract 1

Some more tests:
7
1 1 0
1 6 -9
1 0 -1
1 1000000000 -1000000000
1000000000 -1000000000 -1000000000
1234 987654321 -987654321
100000000 987654321 -987654321