任务描述 建立一个继承体系，List 是基类，ArrayList 和 LinkedList 是其派生类。并且编写实现下述函数并达到如下效果。 ostream& operator << (ostream&os, const List&rhs); 做一个流输出运算符重载，其第二个参数是List的常引用类型。我们知道子类的对象天生可以作为父类类型使用，因此 ArrayList a; LinkedList b; operator << (cout,a); operator << (cout,b); 这上面的调

用会根据对象的实际类型调用对应的重载函数，从而实现输出不同派生类的对象的功能。下面是代码示例：

#include <iostream>
#include <vector>

using namespace std;

class List {
public:
    virtual void push_back(int val) = 0;
    virtual int size() const = 0;
    virtual int operator[](int index) const = 0;
};

class ArrayList : public List {
public:
    void push_back(int val) override {
        data.push_back(val);
    }
    int size() const override {
        return data.size();
    }
    int operator[](int index) const override {
        return data[index];
    }
private:
    vector<int> data;
};

class LinkedList : public List {
private:
    struct Node {
        int val;
        Node* next;
        Node(int v) : val(v), next(nullptr) {}
    };
    Node* head = nullptr;
    int len = 0;
public:
    void push_back(int val) override {
        if (!head) {
            head = new Node(val);
        } else {
            Node* cur = head;
            while (cur->next) {
                cur = cur->next;
            }
            cur->next = new Node(val);
        }
        ++len;
    }
    int size() const override {
        return len;
    }
    int operator[](int index) const override {
        Node* cur = head;
        while (index--) {
            cur = cur->next;
        }
        return cur->val;
    }
};

ostream& operator << (ostream& os, const List& rhs) {
    int n = rhs.size();
    os << "[";
    for (int i = 0; i < n; ++i) {
        if (i) os << ",";
        os << rhs[i];
    }
    os << "]";
    return os;
}

int main() {
    ArrayList a;
    a.push_back(1);
    a.push_back(2);
    a.push_back(3);
    cout << a << endl;

    LinkedList b;
    b.push_back(4);
    b.push_back(5);
    b.push_back(6);
    cout << b << endl;

    return 0;
}

输出结果：

[1,2,3]
[4,5,6]