Hello Codeforces! I dont know why my code is giving me WA

http://www.spoj.com/problems/SUBMERGE/

i just use the articulation point algorithm. but still giving me WA

https://ideone.com/nHXAGO

my code

UPD: I solve it just need an extra Vector in order to mark the articulation point and then count XD

Why this code is giving me WA in 811C - Vladik and Memorable Trip, i used DP the tipical take or not take:

here is my code: 27467986

Hello Codeforces!

I was solving this problem

http://mirror.codeforces.com/contest/810/problem/C

and it gives me a lot of WA, here is my code

27302116

but after that I got an AC, because i just implemented this "+Mod" in this line

-- Sum=(Sum-der)%Mod;

And finally I stay like this

-- Sum=(Sum+Mod-der)%Mod;

27302107

why does it happened?

i have seen in different codes this instruction

#ifdef ......

...

#endif

and my question is what does it mean?

Hi Codeforces Community.

Can any one of you explain me this problem:

http://mirror.codeforces.com/contest/760/problem/C

i read it a lot of times but I did not understand or please explain me the examples how does the process go?

hello Codeforces community, i participated in the "Red de Programacion Competitiva" activity #3 :

https://acm.javeriana.edu.co/maratones/2016/03/

and i would like you to help me in, how can I solve the "Problem B. Tobby Bones":

here is the link of the problems:

https://acm.javeriana.edu.co/maratones/2016/03/filedownload.php?oid=-1&filename=OFlRXXMRQmOnKS1eKONXXl6aKpqbQFcFtUPEmaLLNHkBn%2FcNI%2BgOFZt3gP%2FcdvnyM9fYowpCHgCOC9ijoWEi5LGlnjHDYwqH2XmU4zD4TycQaNrwqFkF6a6CrhGOORUYk64tYtUcim6LBgN2Ju7IV16fZoTNEN45xPJpNYS5mJFhvdmgSoSAuR07wqVtBNWS%2BNWgXyseW0jjEVWd3Si3wA%3D%3D&check=89a3e84eff6c83b371dba559fa475b58f51ff7da3589127492ff6ad7ddb86cbf

Thanks and sorry for my english

Hello Codeforces community, I am in the International Olympiad of Informatics (IOI) and i would like to ask you , how can i improve my logic skills, my Programming Skills, what kind of Algorithm may i know in order to succed in this Olympiad , thaks you , and sorry for my bad English