I want to know what do orthodontist do as an alternative to not pulling teeth for tooth crowding?

Two main ways: Either expand the size of the dental arch or gently slenderize some teeth so they'll fit in the existing arch.

Depends: On the case. We sometimes 'slenderize' nearby teeth ever so slightly to create some extra room. In other cases its a matter of tooth angulation (if for example they are presently retroclined) or, depending on age, palate expansion. All of these are options to avoid extraction, but in some cases the crowding to too extreme to solve by any other means and extraction is required.

All Depends: If the dental crowding is not too severe, minor dental arch expansion may create enough space to alleviate the crowding. The upper jaw can be expanded (as an adult, surgical assistance is normally needed), however, the lower jaw cannot be expanded with or w/o surgery. A complete diagnosis and treatment plan is required to make that determination.

Analysis: A qualified Orthodontic Specialist will do an arch-length analysis, comparing too-size to jaw-size. If teeth can be properly supported over good bone support there are many good non-extraction treatment modalities. If aligning the teeth means moving them off of their bone support, reduction in tooth mass may be necessary. See an Orthodontist for an examination and discussion of available Rx.

Second opinion: Extractions of permanent teeth make orthodontic treatments easier. Get a second opinion. Extracting healthy teeth should not be taken lightly.

Ortho consult yet?: Have you had an orthodontic consultation? The orthodontist will do an analysis of space available and will come up with a viable treatment plan. If you do not feel comfortable with the treatment plan seek another opinion or go to the faculty practice of a dental school.