Compilare: user.cpp: In function ‘void add(std::string)’: user.cpp:17:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < s.size(); ++i) { ^ user.cpp: In function ‘void remove(std::string&)’: user.cpp:32:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (i = 0; i < s.size() - 1; ++i) ^ user.cpp: In function ‘int freq(std::string&)’: user.cpp:39:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < s.size(); ++i) ^ user.cpp: In function ‘int pre(std::string&)’: user.cpp:49:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < s.size(); ++i) ^