Hi. Today competitive programmers certainly have a lot of online resources to practice. Nevertheless, there are so many problems out there, that it's hard to find some "valuable" problem sometimes. By "valuable", I mean a problem that is not just some repeated-idea problem or some problem that the participant already encountered before, or maybe just some problem that provides no new knowledge or it's too long/boring to be worthy to practice, since you will expend more time reading/understanding/coding the problem in relation to what you really learn from it. So, I would like everyone interested to post a small (or big?) list of problems that he/she considers VERY valuable so far in his/her practice so far. If people like the idea, we can later collect this information and compile some nice list of "The Most Valuable Practice Problems" or something similar. Thanks for reading and looking forward to hear the opinion from the community!