Eroare de compilare: user.cpp: In function ‘int valid(int)’: user.cpp:14: error: call of overloaded ‘abs(int)’ is ambiguous /usr/include/c++/4.4/cmath:94: note: candidates are: double std::abs(double) /usr/include/c++/4.4/cmath:98: note: float std::abs(float) /usr/include/c++/4.4/cmath:102: note: long double std::abs(long double) user.cpp: In function ‘void back(int)’: user.cpp:45: error: ‘exit’ was not declared in this scope user.cpp:38: warning: suggest explicit braces to avoid ambiguous ‘else’