I think adding an Update paragraph to your original post would be an effective way to both mark something as solved and to indicate which solution you used. Once a question is posted it partly becomes an opportunity for the Monks to flex their muscles and demonstrate better / faster / stronger solutions. There will almost definitely be numerous solutions in a short order, each with varying degrees of effectiveness and insanity. My problem with a simple "solved" toggle is that it provides no context about which solution you used. Updating your original post can provide that context without requiring another input for the form.