couple of changes

Hi Phil,

Just a couple of changes. The first one is in the initialize method; As we're expecting a hash as a parameter, We need to iterate over values of the hash set and then create a new hash set and add it to children (line 9).

The second change is in the visit_all method, when we call visit all, we need to iterate over children as the tree can potentially have more than 1 value.

Hope this helps mate.

Cheers

Joe