As we know, if someone cheated in a contest, using other contestants' codes, his solution will be skipped.

But, Codeforces allows to use code which was published online before the contest.

So my question is that how Codeforces determines if or if not the code was published online before?

I think I am ok with algorithm problems, but I am so bad in problems like greedy, constructive problems, interactive problems. The problem A, B and C in div2 are often problems like that. I often can't find the key observation. How to practise? Just doing more problems like that or something else?

I was trying to solve 25E, I used hash and passed it, this is my submission. There is also a similar problem in SPOJ, link. The difference is only that the problem in SPOJ has $$$T$$$ test cases, where codeforces has only one.

I clear for every test, but got wrong answer in SPOJ. Is my algorithm wrong or there is some corner cases i didn't realize? Thanks.

I want to solve a problem, which includes three operations: add an edge, delete an edge, and check if the undirected graph is connected. Both online or offline algorithm is ok. How to solve it for $$$n,m \le 10^5$$$?($$$n$$$ denotes the number of vertexs, $$$m$$$ denotes the number of operations.）