Compilare: user.cpp: In function ‘void Back(int, int, int)’: user.cpp:28:37: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0; i < sol.size(); i += 2) ^ user.cpp: In function ‘int main()’: user.cpp:59:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0; i < v.size(); ++i) ^