Count how many different values a list takes in Mathematica

Question

I would like to get the number of different values found in a List.

For example:

The output for the List a={1,2,3,4,5} would be 5 whereas it would b

Answer 1

CountDistinct[a]

would also do the trick, which is a function introduced in Mathematica 10.0, a function equivalent to

Length[DeleteDuplicates[a]]