keep track of pairs changement

Question

Suppose I have a certain number of consecutive numbers to which I assign another number. For example, the following pairs:

0 -> 1

1 -> 2

2 -> 0

3 -> 0

4 -> 1

5 -> 3

Next, I call a function that adjusts the numbers on the right:

0 -> 2

1 -> 0

2 -> 0

3 -> 0

4 -> 2

5 -> 2

Now I need to loop over all the possible numbers that are on the right side (0 and 2 in this case) and do something with them. The problem is that after calling the function, I don't know how many different numbers are on the right side, so now I can't determine the number of iterations of my loop and I also don't know which numbers to loop over. So I don't know which is the most-performant way to keep track of the changes made by the function. Can anyone help me?