I've played with yi ever since I bought him, i use a dps build, and its pretty good.(18 kills, 1 death in one game) but i heard that ap yi was a lot better. could someone please give me an ap yi and tell me why he's better than dps yi?

AP Yi heals himself for a lot more and he does more damage with his Alpha strike. IIRC, a council member build him AP to "show" that AP Yi is OK, but most people play AD/DPS Yi, not AP.

I'm honestly not sure of a build for him, but I would think a Mejai's, Nashor's, and Lich Bane are needed. Probably a Rylai's, and then Guinsoo's and the new Gunblade would be decent. This is completely pulling something out of thin air, and doesn't include boots at all. I would probably get boots and then everything but the Guinsoo's. Again, that's completely out of thin air with nothing to back it up.

^pretty much what he said AP Yi isn't the most common way to play Yi but is still pretty viable. If u want an item build here is wat i go for: rush sheen, then boots that fit the game(i.e. enemy has lot of CC get merc treads, enemy has lot of dps get ninja tabi, and if it doesnt fit in to those get sorc shoes), then turn your sheen into a lich bane, go for zhonyas,( the game is most prolly over by this point), Rylais, Void staff, any item of ur choosing

AP Yi is a machine early on...but seems to become lack luster as the game goes on. Complete opposite with traditional Yi.

This. In the laning phase, AP Yi seems to ruin other champions. Not so great in a teamfight though, and the cooldown on Alpha strike makes it less viable later

AP Yi works extremely well if you have a good team you can count on. If you can rely on your teammates, you can use Highlander + Alpha Strike to get kills, resetting your cooldowns. Then you Alpha Strike again and again, attacking, using your other abilities (especially Meditate and Ignite if you have it) whenever needed. In an idea situation, it's entirely possible for an AP Yi to carry his team just as well as a DPS Yi would, if not better.

AP Yi starts off fairly slow (though faster than a DPS Yi), and you need to farm quite a bit and help your allies get kills/assists in order to get started. Once you have a few key items, though, your Alpha Strike will be one-shotting creep waves and taking off massive chunks of HP from opponents' squishy champions.

A general build would be:

Doran's Ring
Boots
Mejai's Soulstealer [if you can get stacks]
Zhonya's Ring
Lich Bane
Void Staff
Rylai's Crystal Scepter

The last few are interchangeable based on what you need.

I've tried AP yi a few times because it seemed hilarious. I don't know why everyone suggests a Lich Bane though, he doesn't have spammable enough skills to get enough procs imo. I think I'll try a hybrid build soon, good old guinsoos.

Lich Bane allows for a quick 1-2 Alpha, Lich proc combo.

AD Yi and AP Yi are both somewhat viable, at different times, and require much different playstyles.