Hello, I have

If the dog is on the leash ALL the time then yes this is cruel.  If the leash is used a way of confining the dog when you cannot supervise for short periods only in the daytime with access to water and comfortable bedding then I would say it's OK.  At night is also OK because you are giving her regular breaks so she is not forced to soil her resting area.  But having said that I think a crate would be better.  Being tied is less pleasant for the dog.  If the crate is introduced kindly then the dog comes to see it as his "den", a safe place, a bolt hole to retreat to if they are tired, lonely or scared.  The lead doesn't give the dog this benefit.  So my advice is YES acquire a crate as soon as you can.

Ideally you should have known about the dog beforehand and been fully prepared, with time to get a crate made if that was how you wished to train and time to research the breeds you liked so you coul get one that suited you.  But done is done.  Have you had a dog before?  Have you had a Rottie before?  Or any other large, powerful breed with an inherent instinct to guard and protect?  If not then you need to catch up on some "homework" straight away because this is not a breed for the inexperienced or faint hearted and these dogs are frankly a liability if not brought up and trained correctly.
 
Edited to add:  Have yo posted in the Breed discussions section?  Have you looked in the Training section - there is a board there for "house training, crates and puppies".  Do a forum search to get some more info.

as chuffy said, i would say, given adequate exercise and breaks from being on leash is not cruel. i Do think however that it is quite dangerous.. she could strangle herself... i would press on trying to find a crate. when i first got my dogs i thought it would be hard to find crates here n beijing, but a little asking around did the trick and now i have 2 really nice wire crates. alternatively, you could just have one made for the dog..
good luck!

I have to disagree with Chuffy, I don't think the pup wearing a leash around the house is cruel. Having the pup leashed to you is like a walking crate, and is a good way to housetrain. I would just be sure never to leave her unsupervised wearing a collar & leash, because she could get tangled or hung up on something, and I would also give her a break from it when she's sleeping. Confining her to one room with you is also a very good way to housetrain. I think if what you're doing now is working, you don't necessarily need a crate. And remember that a small puppy-proofed room can also be used to "crate" if you need it.

I think Chuff took it to mean that the dog is tied to something when leashed.

Which sounds like the case.

Oh, oh, oh... I get it. Okay, well then I agree with Chuffy that the pup should only be tied up for really short periods if needed - though really I would be uncomfortable even doing that, a puppy can tangle himself up in short order. Anyway, I still think you can housetrain without a crate, just tie the puppy to YOU instead of an object. When you can't be with him, confine him to a small puppy proofed room. If you have to leave for a few hours or more, provide some place for him to potty - newspapers or a litterbox. This is going to mean retraining him off the indoor potty area later but you can only do what the puppy is capable of at any time.