How to find out all array element indices equal to several values (>2)

For example, I have an array a=`[1 2 3 4 5 5 4 3 2 2 2 1]`

, I want to know the indices of all elements equal to b=`[2 5]`

Remember, I cannot use style like `a==b(1) | a==b(2)`

because number of elements in b is arbitrary.

Do I have to use a for loop to do this?