And although it would be more advanced instead of returning null if you can't get a perfect match you should probably buy upto the most you can so
buy(3,[1,1]) = [0,1]

buy(5,[1,1,1,1,6]) = [0,1,2,3]

buy(1,[2,3]) = null

Shouldn't buy(5,[5,2,3,4,5]) return [0] and not [1,2]?

My solution passes but I think it shouldn't because in my tests my solution fails on the following.

Test.assert_equals(sort_string('bansanas', 'bn'), 'bnnasaas')

It might be helpful to mention the basic format of url paramaters (i.e. www.example.com/?key1=value1&key2=value2

Oh cool. Thanks for that info.

Doesn't this give one too many in the list?

I think the point was to fix the switch statement, but this is a cleaner variant.