Compilare: user.cpp: In member function ‘void Trie::add(std::string)’: user.cpp:35:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < c.length(); i++) { ^ user.cpp: In member function ‘void Trie::rem(std::string)’: user.cpp:48:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < c.length(); i++) { ^ user.cpp: In member function ‘int Trie::app(std::string)’: user.cpp:58:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < c.length(); i++) { ^ user.cpp: In member function ‘int Trie::pre(std::string)’: user.cpp:70:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < c.length(); i++) { ^