What is the next number? 12, 21, 23, 32, 34, 43

The pattern seems to be to reverse the number order, add two, then repeat.
34 flipped to 43, so the next step would be to add two--45. 

12, 21, 23, 32, 34, 43,45because 12 flips to 21
23 flips to 32
34 flips to 43
and 45 would flip to 54 i picked this because 12 is 1 and 2 so 23 is 2 and 3 and 34 is 3 and 4 so 45 is 4 and 5

Hello!
volume of sphere =4/3.pi.r^3