Compilare: user.cpp: In function ‘void back(int)’: user.cpp:16:39: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0 ;i < v[k].size(); i++) ^ user.cpp:23:43: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < v[k].size(); i++) ^ user.cpp: In function ‘int main()’: user.cpp:44:13: warning: unused variable ‘rad’ [-Wunused-variable] int rad = sqrt (x); ^