Compilare: user.cpp: In function ‘int main()’: user.cpp:40:25: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(i=0;i<a[n].size();++i) ^ user.cpp:45:29: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(j=0;j<a[i].size();++j) ^ user.cpp:53:29: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(j=0;j<a[i].size();++j) ^