You can find the source code on github:
And the implementations:
A crawler viewer: http://quasark.dorefy.com/crawler/
A balance checker: http://quasark.dorefy.com/check-balance/
All the implementations use the crawler library available on github.
From a public key, you can check and find all transactions/operations of an account. You can also choose if you want to display the data of the stellar crawler into a table or into a tree.