Yesterday I solved problem 1315B. My submission unintentionally coincides with the submission 71715913 made by i_m_swachha, so my verdict got skipped. I checked my code on ideone under default settings. What evidence should I provide to prove that my solution is original?
You can't. If you use ideone under default settings, it's your own fault if someone copies from you. You can't do anything to get it unskipped. Just put it down as a lesson and be smarter next time.
This conversation happens on a weekly basis, I really wish there was some big bold letter sent to everyone that makes it clear beyond doubt to everyone whose solution gets skipped.
My submission came later, so the other person did not copy from me. Even if I use privacy settings, there is a chance that my code might match closely with someone else's code because of a coincidence. What am I supposed to do in such a situation? Simply leave it to my luck that nobody else's style matches my writing style?
Look at the codes. Do you seriously expect that someone independently came up with the same program?
In practice, there is no such chance. For example, I've taken part of over 100 contests and this has never happened to me. I know, what you're thinking of — because the solution idea is the same, the code will also be similar, and thus with 10000 other contestants, someone will have a very similar program. But in reality, even if everyone comes up with the same idea, when it takes at least 50 lines to implement, there are enough variations that even the closest solutions aren't that similar.
I've been involved with plagiarism checking for local contests and yes, the gap between "two codes are copied from one another" and "two codes that look similar" is pretty big.