sth

There is a problem in this file "data_structures/binary_tree/avl_tree.py"

1

In the process of my study, I found that this file can not run successfully every time. ``` File "D:/data_structures/binary_tree/avl_tree.py", line 148, in rl_rotation assert right_child is not None` AssertionError...

### Describe your change: * [ ] Add an algorithm? * [x] Fix a bug or typo in an existing algorithm? * [ ] Documentation change? ### Checklist: * [x]...