有下列程序: #include<iostream> using namespace std; class ONE { public: virtual void f(){COUt<<"1";} }; c1assTWO:public ONE { public:TWO(){cout<<"2";} }; class THREE:public TWO {public: virtual void f(){TWO∷f();cout<<"3";} }; int main() { ONE aa,*p; TWO bb; THREE cc; p=&cc; p->f(); return 0; } 执行上面程序的输出结果是______。