Ok, it may not be "directly against the rules", but it is definitely dicouraged:

    > I learned so much solving problem XXX so is it okay to publish 
    > my solution elsewhere?

    It appears that you have answered your own question. 
    There is nothing quite like that "Aha!" moment when you finally
    beat a problem which you have been working on for some time.
    It is often through the best of intentions in wishing to share our 
    insights so that others can enjoy that moment too. Sadly, 
    however, that will not be the case for your readers. Real 
    learning is an active process and seeing how it is done is a 
    long way from experiencing that epiphany of discovery. 
    Please do not deny others what you have so richly valued yourself.