This is a build to solo top as Master Yi. In my opinion, Master Yi is one of the best in the game at soloing the top lane. I've never lost solo top in solo queue that I can remember and I rarely even lose 1v2. If you play well, you should usually come out of laning phase with plenty of farm as well.

Magic resist/level glyphs - These will help keep you from getting bursted hard by mages and make you a much greater presence in team fights.

Mana regen seals - These are great for early game, so you can heal more and alpha more if you need to. They also allow you to spam more abilities in team fights, when your ult is allowing you to reuse your abilities.

Armor penetration quints/marks - These make you much stronger early game. Then, along with the Youmuu's Ghostblade, they allow you to play without needing to buy a Last Whisper or Black Cleaver.

Yi's a hardcore DPS champ; the attack tree will benefit him the most. The utility tree is also viable on him though. I go the armor/magic resist runes for more survivability, so you can take more hits without dying.

Teleport - I get this every time on Master Yi. It's great early game for soloing top, so you never need your jungler to cover for you. It can also be used to gank the other lanes, if they place wards. On a strong dps like Yi, it gets even better as the game progresses. Teleport is the ideal spell for backdooring and Master Yi is quite likely the best backdoorer in the entire game. It's especially helpful if you can get some wards near the enemies' turrets/base to teleport to.

Clarity - Call me crazy, but I'm a fan of clarity on Yi. This makes him the ultimate sustain champ early game. It also allows you to constantly spam abilities in team fights, which is great if you're getting kills with your ultimate. However, if lane with a support that can give you mana or if someone else on your team takes clarity, you can also take Flash, Exhaust, or something else.

Doran's Ring - Mana regen, ability power, health are all a great start for yi. Thanks to your W, you don't need health potions, so you may as well spend your whole 475 on a ring.

Sheen - More mana and ability power is great for early game yi. The passive is amazing on him all game too.

Berserker's Greaves - These are best if you just want damage. Sometimes, you may need Merc's Treads instead though, especially if you're getting focused by cc in team fights (which is very likely).

Youmuu's Ghostblade - CDR is great early game for your Q,W spam. It also gives you a great passive for chasing and killing.

Vampric Scepter - Keeps you alive in team fights and helps so you don't have to rely as heavily on your W. This is definitely a great on Yi and should be gotten early, so you can win more 1v1s sooner.

Infinity Edge - The damage, crit chance is all great. The passive is amazing as well.

Phantom Dancer - Movement speed and crit chance makes you a much greater threat.

The Bloodthirster - Damage and life steal. This can be replaced with a Stark's Fervor. I prefer Bloodthirster though, because the damage makes your first attack after alpha a lot stronger.

Trinity Force - Build the Sheen into this. It gives a little of everything and is generally very good. The best part of it though is easily the on-hit effect after using your alpha.

Second Phantom Dancer - With the Phantom Dancer, Trinity Force, and Youmuu's passive, plus another Phantom Dancer, you should have no need of boots. No boots ftw.

Another option to get, rather than the Bloodthirster, is a Hextech Gunblade. It's a bit more expensive, but is arguably a better item for Yi. Spell vamp for your alpha is great, ability power is great for your Q and W, and it still gives you the life steal and AD. Also, it has an awesome active for killing.

Stark's Fervor is another decent option. I would only get this if there are other AD champs on your team that would benefit from the life steal.

2nd Bloodthirster/Hextech Gunblade - If you need more life steal, go for it.

Frozen Mallet - An extra slow helps a lot so you don't have to chase as far. The health is also a big help in team fights.

Banshee's Veil - It has an epic passive. The health and mana is pretty ok too.

Guardian Angel - Armor/MR, passive. It's a good item if you need help staying alive.

Almost anything else - AP, AD, and tank items all help a Yi. So, you can really get by with building just about anything on him.