Why test cases and source codes are not visible after gym contests?

Can someone explain how to go through this problem http://www.codechef.com/problems/A6. I have found this solution but i can't understand its precomputation part. http://www.codechef.com/viewplaintext/2279326

http://www.spoj.com/problems/CALCULAT I have been trying solving this problem, I have first tried to calculate first k digits of factorial using stirling approximation but due to precision my code don't give perfect ans for all factorials and I cant figure out how to find last l digits of factorial. Can someone tell me how to crack this problem. Thnks.

where I can solve icpc 2013 world finals problems?

Can someone provide me some resource or code for fast hashing that can be used in topcoder SRM's/Codeforces rounds. OR I have to use set/map for hashing purposes.

Suggestion Welcome For: In what kind of problems hashing to be used.

http://mirror.codeforces.com/contest/304/problem/E

how to set problem as favorite and access after? So that left problem can be tried later like, in spoj when some problem we attempted is not yet submiteed it goes under TODO problem.