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(int, std::string&, trie*)’: user.cpp:30:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if (k < s.size() - 1) ^ user.cpp: In function ‘int freq(std::string&)’: user.cpp:42:32: 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:52:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < s.size(); ++i) ^