I personally use CodeWars to practice my thinking, not "using libraries which does my job". While in office I would search for solution in external libraries but it does not have any point in here

Use it if you want. It's just a tool among others.

As for what would make it best practices or not... Here, it's certainly not, since it makes the algorithm O(N²) (and yet, it might very well be faster than a proper linear solution, unless N grows very large... x) )