Compilare: user.cpp: In function ‘bool valid(std::vector<int>)’: user.cpp:19:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0; i<v.size(); i++) ^ user.cpp:20:33: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int k=i+1; k<v.size(); k++) ^ user.cpp: In function ‘void write(std::vector<int>)’: user.cpp:29:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int k=0; k<v.size(); k++) ^ user.cpp: In function ‘void generate_permutations(int)’: user.cpp:49:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if(vec.size()==n) ^