* Fix insertion fail in ["*X", "*XX"] cases
Consider a word, and a copy of that word, but with the last letter repeating twice. (e.g., ["ABC", "ABCC"])
When adding the second word's last letter, it only compares the previous word's prefix—the last letter of the word already in the Radix Tree: 'C'—and the letter to be added—the last letter of the word we're currently adding: 'C'. So it wrongly passes the "Case 1" check, marks the current node as a leaf node when it already was, then returns when there's still one more letter to add.
The issue arises because `prefix` includes the letter of the node itself. (e.g., `nodes: {'C' : RadixNode()}, is_leaf: True, prefix: 'C'`) It can be easily fixed by simply adding the `is_leaf` check, asking if there are more letters to be added.
- Test Case: `"A AA AAA AAAA"`
- Fixed correct output:
```
Words: ['A', 'AA', 'AAA', 'AAAA']
Tree:
- A (leaf)
-- A (leaf)
--- A (leaf)
---- A (leaf)
```
- Current incorrect output:
```
Words: ['A', 'AA', 'AAA', 'AAAA']
Tree:
- A (leaf)
-- AA (leaf)
--- A (leaf)
```
*N.B.* This passed test cases for [Croatian Open Competition in Informatics 2012/2013 Contest #3 Task 5 HERKABE](https://hsin.hr/coci/archive/2012_2013/)
* Add a doctest for previous fix
* improve doctest readability
* added radix tree to data structures
* added doctests
* solved flake8
* added type hints
* added description for delete function
* Update data_structures/trie/radix_tree.py
* Update radix_tree.py
* Update radix_tree.py
* Update radix_tree.py
Co-authored-by: Alex de la Cruz <alex@Alexs-MacBook-Air.local>
Co-authored-by: Christian Clauss <cclauss@me.com>
