The same one as everyone else. Almost everyone nowadays has been using computers since they were little, and thus they just use the same scheme as everyone else. I've never used a left-handed mouse, I use the same keyboard scheme as everyone else and I've never had to reconfigure buttons for left-handedness. I've been using computers since I was really little so that's just natural to me, and that's how most left-handed people are. The only left-handed computer thing I do is that I use my left hand for the trackpad on my mac, and if anything this is an advantage because ctrl+click is the right-click and I can use my pinky to hit it. My brother fussed at me for not having my mac set up for an alternate right-click and I didn't get why until I realized the only ctrl button is on the left and so I could hit it with my pinky and he couldn't. But still this doesn't affect how you'd set up a game, so yeah don't worry about the lefties unless you're making a DS game.
Also I don't think anyone really uses Dvorak keyboards. Don't worry about them either unless one contacts you about it.
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