Locking a door without changing the door knob?

[Vantage]

So this is a bit of a random question, but does anyone know of a non-destructive device (or thing, not sure the right word to use) to keep a door locked,
1. Without changing the door knob/handle AND
2. Without causing any permanent damage to the wall?

I'm going to be living in a place where the two bedrooms are attached by a bathroom. And while you can lock both doors from in the bathroom, you can't lock the room from the bathroom, so the other room could just come in when you're sleeping or whatnot.
Unfortunately, I can't drill or hammer anything into the door frame or wall or change the knob (and please don't suggest doing it and then doing repairs later; trust me, I cannot do anything to these), so I need something that can help me lock the door from the room (or at least keep it from opening) without causing damage.

Any good ideas?

[Nemmerle]

Stick a length of wood or metal up under the doorknob and brace it back against the floor in a triangle. Preferably with something fairly heavy wresting at the end that goes into the floor so it won't creep forward if smacked hard.

[Rich19]

Put a chair or table or something like that under the door handle? Never tried it myself, but they do it in films...

[Mastershroom]

Nem and Rich are right; wedging something large and heavy between the floor and doorknob is pretty effective and hard to brute-force. If you have two large heavy objects to spare, like two desks or something, you could put one on either side of the door, and some firm beam or metal or wood between them across the doorway.

Actually, in retrospect...does the door open inward (i.e. towards your room) or d (towards the bathroom)?

[Vantage]

Toward the room. And it would be kind of annoying to have to move a desk each time I need to use the bathroom. :-/
But just something heavy might be the best solution. I had certainly thought of that it just seemed... low tech.

[Red Menace]

Just a regular door with a regular lock? Why can't you just turn the lock around so that it locks from the bedroom side and keep it locked? Super easy to do, I did it at my girlfriend's apartment because she had the same issue. So you are not really changing the lock, just flipping it around and flip it back around when you leave.

[SuperSmeg]

Pay a fat guy to sit by the door, blocking it.

...

Hey, whats with the funny looks?!?

[Vantage]

Originally Posted by Red Menace

Just a regular door with a regular lock? Why can't you just turn the lock around so that it locks from the bedroom side and keep it locked? Super easy to do, I did it at my girlfriend's apartment because she had the same issue. So you are not really changing the lock, just flipping it around and flip it back around when you leave.

This is school housing. I think I'd get flak for that. That's why I really can't do any modifications.

[VFrieden]

You could lock the other guy's door (I'm assuming he can't lock from his side, either) and keep the restroom for yourself

[Inyri Forge]

Originally Posted by Vantage

This is school housing. I think I'd get flak for that. That's why I really can't do any modifications.

Is it a dorm or rental housing? If you're having a security issue and the housing is provided by the school, ask the school to make the modification.