Firstly, these questions don't really get the same kind of feedback as the other questions.
Most questions on this site get feedback on style. Ways are suggested to make code more readable, follow conventions, etc. People asking these questions neither want nor need advice on improving their style. Style simply doesn't matter for these kind of questions. As a result, a large portion of what would normally be good input on code doesn't apply.
Most of the performance improvements suggested on this site fall into the category of avoiding stupid things. This is not universally true, some of the answers do delve away into coming up better algorithms. But in doing so, they really aren't reviewing code anymore. If your developing better algorithms you aren't really doing a code review anymore. Its not necessarily wrong to provide a better algorithm, but its a very different focus from the other questions.
Secondly, the code in these questions seems superfluous. They are all just an implementation of the brute force solution to the problem. The correct answer to these questions doesn't involve the code at all. Rather, the correct answer points out where they need to look for a better approach. These questions then aren't really about the code, and this site is about the code.
Thirdly, the asker isn't required to show effort. It looks like they've made effort because they've produced a naive solution. But in the case of contest solutions, the naive effort doesn't count for anything. What we should be asking is whether the asker has made any attempts to come up with a better algorithm.
In summary: these questions require noticeably different answers then other code review questions. They aren't really about the code. They don't show effort on the part of the asker. In my judgment, these questions ought to marked off-topic and closed on this site.