Compilare: user.cpp: In function ‘void bk(int)’: user.cpp:14:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if (k==a.size()) ^ user.cpp:16:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i=0; i<a.size(); i++) ^ user.cpp:21:30: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (a[k]=0; a[k]<a.size(); a[k]++) ^ user.cpp: At global scope: user.cpp:25:6: warning: ISO C++ forbids declaration of ‘main’ with no type [-Wreturn-type] main() ^