Compilare: user.cpp: In function ‘void insert(std::string)’: user.cpp:28:29: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < s.size(); ++i) ^ user.cpp: In function ‘void del(std::string)’: user.cpp:41:29: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < s.size(); ++i) ^ user.cpp: In function ‘int query(std::string)’: user.cpp:52:29: 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:65:29: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < s.size(); ++i) ^