This solution was submitted when a different compiler version and different library versions were used on Codewars. While the concept of modules is gradually introduced in C++, tradicional includes just insert contents of a file into another file. Since some files include other files and some files are included by the testing framework as well, lots of things end up declared in the scope transitively. Then if you rely on things being declared for you, it will break sometime or even already now with just a different compiler. Note that bit/stdc++.h isn't a part of any standard, so if you decide to use it, you should at least know what you're doing.

Should be part of "algorithm" I think. Not sure how it used to work.