Trundle is a really strong early/mid game jungler because he has awsome sustain, mobility and great ganking preasure. He is also decent in the counterjungling aspect for stealing buffs and small camps (only big creep).
I play trundle as an aggressive jungler which is the reason why I don't buy an early Frozen Mallet.
If you want to go for mallet instead of Trinity Force for a more tanky build go ahead!

Pros:

+ He is a troll
+ Fast jungler.
+ You NEVER need to buy health potions in the jungle.
+ Great Ganker!
+ One of the best early game 1v1 champions.
+ Great escapes!
+ Can solo dragon at a really low level if u have purchased a Wriggle's Lantern

Cons:

- No Area of effect damage.
- A really weak passive tho Spirit Visage buffs it alot! (It also works great with Trundles ultimate!)

Ghost: Is a great spell for Trundle in my opinion because IF your Contaminate is on cd you can easily chase your enemys, also works great for escaping.
Flash: Also works great for chasing/escaping over walls etc.
Smite: Is a must while jungling with EVERY champion, It's also needed to secure Buffs/Dragon/Baron for your team!
Exhaust: Is also a great spell for chasing I prefer it ONLY on Lee Sin, Dr. Mundo, Shyvana, Riven and Jarvan IV depending on the situation, It's pretty usefull on trundle aswell since you have alot of escapes already...
Ignite: Can be usefull in some situations but I suggest not to take it, Only if facing Champions like Garen/ Dr. Mundo/ Tryndamere/ Volibear to prevent their ultimates and Garen/ Volibear/ Dr. Mundo insane health regeneration, IF your AP mid/AD carry for some reason haven't picked it up.

With this setup of items you can easily solo baron nashor to help your team out aswell as beeing that offtank jungler which ur team might need especially in solo queues!

Core items:

Wriggle's Lantern Trinity Force Randuin's Omen Mercury's Treads Frozen Mallet Spirit Visage

Situational items:

Force of Nature Guardian Angel Frozen Heart Warmog's Armor Madred's Bloodrazor Wit's End


Vampiric Scepter
Is probably the best item to start with on trundle while jungling from my experience. Because you wont EVER need a health potion aswell as it's a part of wriggle's lantern which is your most important early-game item for jungling as an AD champion IMO. (Makes you able to solo dragon at level 5-6 without losing more then half of your health.)

Mercury's Treads
Is my favorite pair of boots because of the CC reduction it gives you + the magic resist is great if your facing alot of AP based champions. Another good choice is Berserker greaves or Ninja tabi's for the attack speed or melee damage reduction aswell as some armor. Depends on what your after!

Wriggle's Lantern
Is the most important Item for an AD based jungler except tanks. IT'S A MUST! (Except on some champions like Shaco you can go for a Mardred's Bloodrazor instead)

Trinity Force
I pick up an early Trinity force because of its incredible damage output early-game + for its mobility. Can be swapped with Frozen Mallet!

Oracle's Elixir
If the enemy team is warding alot I find it to be a waste to let the support buy alot of Vision wards/Oracle since your already running around ganking. And its great if your going to take early Dragons or if your ganking alot you can clear their wards.

Sight Ward
REALLY usefull to place out while your jungling, on buffs, gank spots pretty much just help out your team in general, because your support have to focus on bot-lane... I'ts great to pick up around 2-3 of them after you've got Wriggle's Lantern and Mercury's Treads!

Spirit Visage
Works amazingly well with your ultimate and passive great mid-game item imo.
Can be swapped with Bloodthirster!

Wit's End
Is a great cheap item for Trundle if ur facing a magic heavy team. Wit's End Phage is probably the most broken item combination at the moment.

Randuin's Omen
Is really strong against ad heavy teams also built from a Heart of Gold can be swapped for a Frozen Heart.

Atma's Impaler
Is a relativly cheap item and it's amazing overall gives you tankyness aswell as damage and crit! A core item in my opinion...

Madred's Bloodrazor
A really sick item for trundle as it gives attack speed, armor and damage plus a passive which helps if your facing a really tanky team late-game! can be swapped for a Wit's End.

Seals:
I take Greater Seal of Armor seals to have a more smooth jungle route!

Marks:
I take Greater Mark of Desolation For some early game damage, Feel free to switch them with Greater Mark of Attack Speed or Greater Mark of Attack Damage.

Glyphs:
I take Greater Glyph of Magic Resist because I really like the flat magic resist on any AD based champion I'ts also great with some early magic resist to gank mid lane. (You can also take magic resist/level if you fancy that more!) Feel free to switch them with Attack speed or CD reduction glyphs!

Quintessence:
I take Greater Quintessence of Movement Speed for a really smooth jungle and to have alot more mobility early game.. I find it more usefull then + ~ 9 Attack damage or flat health.

The standard jungle route for Trundle is Wolves -> Blue buff -> Wraiths -> Red buff -> Double golems etc, etc...

If you are more into the Counter-Jungling side of jungling I suggest going Wolves -> Blue buff (without smite, with help from bot/toplane) -> Steal enemy Red Buff into a top/bot lane gank...

You can also start Doran's Blade or Boots 3x Health Potion and start at Wraiths -> Red Buff (Take Pillar of Filth at lvl 2) -> Mid/top Gank if you fancy early ganking.