I'm just curious, is it normal for the hard puzzles to require trial and error to solve? I've only come across three or four so far that I've needed to do that, but I'm curious if I've missed something. Thanks
Most "easy" soduku can be solved by direct, logical deduction (that's why they're easy )
Harder ones require you to narrow a puzzle down to a manageable number of contingent cases and then test these cases. Here, you're technically "guessing", but in a methodical way.
If you're really interested, there's a veritable War & Peace on the Yea & Nay of Trial & Error already written in this forum - it does take a bit of time to search through it - but it's fascinating reading!
Also read the 'Three Features of All Pappacom Puzzles' in the General/Program section of this forum.