Problem 199. hanoi towers II

with 3 rods (1,2 & 3), the goal is to move a tower of n disks from rod #1 to rod #3. You can move disks only one by one, on the rods. You can't put a disk on a smaller one.

At the beginning, all disks are on rod #1, biggest at the bottom, smallest at the top.

Give the smallest sequence of unused rods for a n disks Hanoi tower problem.