### kittyK's blog

By kittyK, history, 7 weeks ago,

Can anyone explain how to solve this 687B - Игра с остатками . I don't understand how to come with ideas for such problems and I did not understand the editorial well.

• +2

 » 7 weeks ago, # |   0 If you know xmodc1,xmodc2,...,xmodcn then you can find out xmodlcm(c1,c2,...,cn) if the given values are not self-contradictory(eg. xmod2=0 and xmod4=1). Conversely, if you know xmodlcm(c1,c2,...,cn) you can find each of the given n values. So first find xmodlcm(c1,c2,...,cn) and this would retain all the information. Now, if k divides lcm(x1,x2,...,xn) you can easily find xmodk otherwise you can't.
 » 7 weeks ago, # |   +10 The editorial proves that all you have to do is check whether $k \mid \text{lcm}(c_1, \dots, c_n)$. This is equivalent to saying $\gcd(k, \text{lcm}(c_1, \dots, c_n)) = k$. But we can calculate the left side very easily since $\gcd(k, \text{lcm}(c_1, \dots, c_n)) = \text{lcm}(\gcd(k, c_1), \dots, \gcd(k, c_n))$. The time complexity will be $O(n(\log(C) + \log(K)))$. Also there will not be overflow because the intermediate value will always be $\leq k$.These identities hold because if you think of an integer's prime factorization as a multiset, $\mid$ corresponds to $\subseteq$, $\gcd$ corresponds to $\cap$, and $\text{lcm}$ corresponds to $\cup$