Mountain Biking - Any good MTB that are also good on the road

Im looking for a hardtail bike, that has front suspension, and is also good on the road, bc i may do some triathalons, and i dont want to guy 2 bikes.

sorry if this is the wrong section (new to this site)

Look into a MTB with 29" wheels. The larger wheel is the same size as a road bike's wheel (700c) and therefore can take a variety of skinny lightweight road tires, and it rolls faster on the pavement. Most will also have a fork with a lockout so you aren't having that energy robbing suspension bouncing all over the place. I recently did a 200mi trip on one with semi-knobby skinny tires and a 30lbs load and managed to ride at 16-18mph on the paved sections, and it still handled great in the dirt.

Look into a MTB with 29" wheels.

Actually you can fit a 700c road wheel with scrawney little road tire on a 26" MTB. The 26" MTB is made to have extra clearance for larger diam. tires and mud. The 700c wheels and tires just barely clear the fork and frame which should be plenty for the road. The only drawback is you will need to run some sort of disc brake b/c of the rim position issue.

So, for your trail riding you can run 26" wheels with Fat Tires and when you do your triathalon you can throw on the 700c road wheels and tires.

so is the k2 Zed 2.0 a good choice( its under 500 whihc is about my budget, and its sold at REI)? how do you know if it can lock its suspension?

Most hardtails will be fine on the road- but look for V-Brakes and a cheap Rigid fork you can fit
I do a few roadrides and have 80mm suspension set up on the firm side in any case so don't bother with the rigid forks. On the tyres- go for a high pressure slick like Cont Grand Prix's at 110psi.

I use my Kona Blast mostly on the street, it's quite capable of handling both easily. The riding geometry isn't the greatest for the road, but for short hauls under 20 miles it's fine.