I think we should check all possible community cards. And for each variant we should find the probability that each player has a combination worse then the first player. This problem can be simply reduced to two-player situation, but I have no idea how to solve it fast.

I've found a DP solution, but it gets WA#11.
Maybe it's because of accuracy, I use pow(x, n-1) in
my program and don't know how to avoid it.
Please, give me a hint or at least some large test!

Precision should not be a big trouble in this problem. At least on this test. Maybe you make some assumptions about the combinations that are not always true.

I don't make any assumptions, I just forgot to sort them))
Nice problem! Now I'm going to speed up my solution to 0.1 sec.