useful to solve graph https://eddmann.com/posts/depth-first-search-and-breadth-first-search-in-python/

MacBookPro:test-checkio denis$ export PYTHONPATH="${PYTHONPATH}:/Users/denis/PycharmProjects/test-checkio" MacBookPro:test-checkio denis$ echo $PYTHONPATH :/Users/denis/PycharmProjects/test-checkio MacBookPro:test-checkio denis$ python3 -c "import sys; print(sys.path)" ['', '/Users/denis/PycharmProjects/test-checkio', '/usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/lib/python37.zip', '/usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/lib/python3.7', '/usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/lib/python3.7/lib-dynload', '/usr/local/lib/python3.7/site-packages']