Oracle how to combination and count two columns : i need help 

my table like this;

A.........................B

1.......................100

1.......................102

1.......................105

2.......................100

2.......................105

3.......................100

3.......................102 

i want output like this:

A......................Count(B)

1...................... 3 

1,2.................... 2

1,2,3.................. 3

2...................... 2

3...................... 2 
how can i do this?

i try to use listagg but it didnt work. sorry for my  english                                 