Funnily enough, the non-array formula in B8 is not the fastest one. The array formula in B5 is faster, also the one in B6 (but this is a tick slower than B5).
The formula in B7 would have been my favourite one if Excel© had a worksheet function “REVERSE”. Then we could just say
Please note that only the formulas in B5 and B6 return correct results for negative inputs or inputs with decimals. The other one assume positive integer inputs.