Compilare: user.cpp: In function ‘void back(int)’: user.cpp:17:40: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0; i < V[j].length(); i++) { ^ user.cpp:21:30: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if(s.length() == N) { ^ user.cpp:23:34: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] else if(s.length() < N) back(k + 1); ^