what kind of hat are you knitting?
there are lots of hat styles!
but --really many hats can be knit flat and seamed, or in the round.
and many patterns can be converted to flat (from round) or to round (from flat)
Flat knit hats have 2 extra stitches. (these will get 'lost' in the seam)
flat knit hats (vs round knit hats) need alternate rows of purls if the pattern is basically a stocking knit
(and round knit hats (from flat patterns?--2 few stitches (no seam and no 2 stitches need for seam allowance)
and round knit hats are worked in all knits (for stocking knit)
Nicky Epstein almost always knits her hats flat.. she doesn't have too many free patterns out there.. but lots of books have her patterns --look (search on ravelry to find images/sources) for her patterns if you want a flat knit and seamed pattern.