Regarding the C++ version: instead of copying data to a vector and then sorting the vector, it might be more profitable to use std::multimap<size_t, string>. This is a sorted container (likely rb-tree based) that handles duplicated keys.

Also, as somebody else mentioned, iostreams are slow. Even simple reading of file can be several times slower than plain C (http://0x80.pl/notesen/2019-01-07-cpp-read-file.html).