If someone touches you inappropriately (and clearly on purpose), what do you do about it?

I was on a bus, just getting on and walking through the isle, and a guy literally just stuck his hand up my dress and grabbed my ass. I couldn't believe it, I mean seriously? I immediately shoved him away, but I didn't even know what to do because right after he ran off the bus. It's so crowded I couldn't even see him then.

Can you file a report with the police for that? I feel like it's useless since I don't even know his name. Should I have slapped him, decked him or something? I'm not a violent person, and I was too shocked. It just really sucked, and I feel awful and disgusting.
If someone touches you inappropriately (and clearly on purpose), what do you do about it?
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