Compilare: user.cpp: In function ‘void expand()’: user.cpp:14:56: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while( a[ r + 1 ] == a[ r - l + 1 ] && r <= a.size() ) ^ user.cpp: In function ‘int main()’: user.cpp:26:29: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for( i = 1 ; i < a.size() ; i++ ) ^