For rolling hash, how do you decide which MOD to pick/ whether to use a tuple of 2 or more hashes? It is hard to know a priori whether the mod is high enough to prevent collisions / too high to cause memory limit / and multiple hashes could cause time limit. In This, 10**9 + 7 gave an incorrect answer, while 10**18 + 3 was accepted. Using a pair of hashes with 10**9 + 7 and 998244353 gave memory limit exceeded.