Eroare de compilare: user.cpp:5: error: variable ‘std::ifstream f’ has initializer but incomplete type user.cpp:6: error: variable ‘std::ofstream g’ has initializer but incomplete type user.cpp: In function ‘int valid(int)’: user.cpp:20: error: ‘abs’ was not declared in this scope user.cpp: In function ‘void back(int)’: user.cpp:30: warning: suggest explicit braces to avoid ambiguous ‘else’