Fix mypy error at maths (#4613)

* Fix mypy errors for maths/greedy_coin_change.py

* Fix mypy errors for maths/two_sum.py

* Fix mypy errors for maths/triplet_sum.py

* Fix the format of maths/greedy_coin_change.py

* Fix the format of maths/greedy_coin_change.py

* Fix format with pre-commit