Refactor code to remove 'using namespace std'

[evotodi]

This PR refactors the code to not use 'using namespace std' as doing so can cause some unwanted effects. While it is very common to use 'using namespace std' I have found that it can cause some headaches when including libraries that use 'using namespace std'.

• https://www.geeksforgeeks.org/using-namespace-std-considered-bad-practice/
• https://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice

[edrosten]

Ah yes I was a bit lazy writing this!