Compilare: user.cpp: In function ‘int main()’: user.cpp:17:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0;i<a.size();++i) ^ user.cpp:23:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int j=0;j<c1.size();++j) ^ user.cpp:24:35: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int k=0;k<c1.size();++k) ^ user.cpp:42:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0;i<c1.size();++i) ^ user.cpp:49:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0;i<c1.size();++i) ^