以下程序的输出结果为( ) #include class Sample { int n; public: Sample(){} Sample(int i){n=i;} friend Sample operator-(Sample &,Sample &); friend Sample operator+(Sample &,Sample &); void disp(){cout<}; Sample operator-(Sample &s1,Sample &s2) { int m=s1.n-s2.n; return Sample(m); } Sample operator+(Sample &s1,Sample &s2) { int m=s1.n+s2.n; return Sample(m); } void main() { Sample s1(10),s2(20),s3; s3=s2-s1; s3.disp(); s3=s2+s1; s3.disp(); }