Compilare: user.cpp: In function ‘void FFT(std::vector<std::complex<double> >&, bool)’: user.cpp:22:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i=0;i<A.size();i++) if (i<rev(i,lg)) swap(A[i],A[rev(i,lg)]); ~^~~~~~~~~ user.cpp:24:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int len = 2; len <= A.size(); len<<=1){ ~~~~^~~~~~~~~~~ user.cpp:27:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i=0;i<A.size();i+=len){ ~^~~~~~~~~ user.cpp:37:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i=0;i<A.size() && inv;i++) A[i] /= A.size(); ~^~~~~~~~~ user.cpp: In function ‘std::vector<int> mul(std::vector<int>, std::vector<int>)’: user.cpp:58:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i=0;i<A.size() + B.size() - 1;i++) ans.push_back(_C[i].real() + 0.5); ~^~~~~~~~~~~~~~~~~~~~~~~~ user.cpp: In function ‘int main()’: user.cpp:86:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i=0;i<ans.size();i++) if (ans[i]) g<<i<<' '<<ans[i]<<'\n'; ~^~~~~~~~~~~