There are three reasons why I think Project Euler etc. questions shouldn't be posted here.
The first is copyright. The question submitter
... represents, warrants and agrees that it will not contribute any Subscriber Content that (a) infringes, violates or otherwise interferes with any copyright or trademark of another party ...
(StackExchange Terms of Service).
The second is that Project Euler strongly discourages people from seeking help elsewhere: the correct place to compare solutions to PE problems is in the forum which you unlock by submitting a correct answer. See Will Jagy's report of discussion with PE about questions posted on math.stackexchange.com.
The third is common courtesy. Ripping off other sites isn't nice.