Mostly, I was just somewhat stupid in this contest.
I finished problem A & B in a short period of time, then got stuck in problem C.
To problem C, I was too eager to find a mathematical based optimal solution so that I didn't think of the clear optimization of DFS/BFS.
After a long struggle, I used the brute force & observation method, getting accepted easily. Although it's somewhat stupid. But, if you can get good score, who cares if you found the proof of a pattern? It was just a failure in strategy.
Wasting too much time on problem C, I had no time to code problem D.
When system test, my problem B just crashed just because a typo of I/O.
Anyway, I love the problemset, nice problems.
Looking forward to the next contest :)