class Sample private: public: Sample(int i) ( n= i;} ___int add(Sample& s1, Sample& s2); int add(Sample& s1, Sample& s2) return s1.n + s2.n: int main@ f Sample s1(10), s2(20); cout <<___<< endl; return 0:横线处需要补充什么

需要补充 `add(s1, s2)`。完整的代码如下：

class Sample {
private:
    int n;
public:
    Sample(int i) { n = i; }
    int add(Sample& s1, Sample& s2);
};

int Sample::add(Sample& s1, Sample& s2) {
    return s1.n + s2.n;
}

int main() {
    Sample s1(10), s2(20);
    cout << s1.add(s1, s2) << endl;
    return 0;
}