Utah did, or so they claim: http://www.npr.org/2015/12/10/459100751/utah-reduced-chronic...

Easy enough to implement "housing first" where housing is cheap.

Housing would be a lot cheaper in SF if SF wanted cheaper housing.

Yes, they have geographical limitations on how much housing they can build horizontally, but on top of those they also have strict self-imposed limitations on how much they can build vertically.

100% solved hardly exists, but is there any city where the problem isn't smaller than in SF?

Maybe Los Angeles? Doubt it, though.