Compilare: user.cpp: In function ‘int main()’: user.cpp:58:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0 ; i < s.size() ; ++ i) ^ user.cpp:61:36: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int j = 0 ; j < s.size() ; ++ j) ^ user.cpp:69:36: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0 ; i < s.size() ; ++ i) ^ user.cpp:71:44: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int j = 0 ; j < s.size() ; ++ j) ^