what, the Editorial is so greatful.But I think this problem is a combinatorics problem. And use inclusion-exclusion principle let~s define var "all" be the number of the total number which occuer twice. let`s define var ans be the total number of permutation of all the number. it can calc it A(n, n) / (all * 2!) and now we use inclusion-exclusion principle. for (k := 1 to all) mean that there are k pair number break the problem condition so we should add the number when k is even and cut the number when k is odd

I don't understand how to come equation A to equation B that are selected red colour in this image? Can any one help me?

@ma5termind I am trying to understand your tutorial. I had to revisit counting, permutation and combination concepts so I kinda understood the explanation part.
Regarding the code part: I'm guessing "inv[i]" means (1/F[i]); where F[i] is the factorial. So, inverse factorial. However, I don't understand how (F[i] ^ (10^9 + 5)) will invert it... or is it a trick for integers (4 bytes). Edit: If anyone else sees this, then also let me know...

can someone explain the recurrence?

pls somebody explain dis prlm....???how to do this