Compilare: user.cpp: In member function ‘std::string ExpressionConverter::convert(std::string&)’: user.cpp:36:36: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < infix.size();) { ^ user.cpp:40:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while (i < infix.size() && isdigit(infix[i])) { ^ user.cpp: In member function ‘int Evaluator::evaluate(std::string&)’: user.cpp:101:38: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < postfix.size(); ) { ^ user.cpp:107:33: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while (i < postfix.size() && isdigit(postfix[i])) { ^ user.cpp:99:9: warning: unused variable ‘result’ [-Wunused-variable] int result = 0; ^ user.cpp: In member function ‘Node* ExpressionTreeBuilder::build(std::string&)’: user.cpp:140:38: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < postfix.size(); ) { ^ user.cpp:146:33: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while (i < postfix.size() && isdigit(postfix[i])) { ^ user.cpp: In member function ‘int Evaluator::operation(int, int, char)’: user.cpp:95:3: warning: control reaches end of non-void function [-Wreturn-type] } ^