Better running time for remove_dup

[apatruni]

Linearithmic solution to remove_dup with linear space. Existing implementation uses quadratic time approach.

[xtaci]

thanks for the PR, but the purpose for this repo is not for "Performance" but "Clearness" for understanding.

[apatruni]

is the function working properly if i pass three consecutive elements which are the same value? for example in the array {'a','a','a'}