编写程序link.h实现例9-6的链表类。在测试程序lab9_2.cpp中声明两个整型链表A和B，分别插入5个元素，然后把B中的元素加入A的尾部。

以下是link.h的程序代码：

#include <iostream>
#include <cstdlib>
#include <ctime>
using namespace std;

struct node {
    int data;
    node *next;
    node(int x, node *t = nullptr) {
        data = x;
        next = t;
    }
    node(): next(nullptr) {}
    ~node() {}
};

class link {
private:
    node *head, *tail;
    int len;
public:
    link() {
        head = tail = nullptr;
        len = 0;
    }
    ~link() {
        while (head != nullptr) {
            node *p = head;
            head = head->next;
            delete p;
        }
        tail = nullptr;
        len = 0;
    }
    int length() const {
        return len;
    }
    bool empty() const {
        return head == nullptr;
    }
    int front() const {
        if (empty()) {
            cerr << "List is empty!" << endl;
            exit(EXIT_FAILURE);
        }
        return head->data;
    }
    int back() const {
        if (empty()) {
            cerr << "List is empty!" << endl;
            exit(EXIT_FAILURE);
        }
        return tail->data;
    }
    void push_front(const int& x) {
        head = new node(x, head);
        if (tail == nullptr) {
            tail = head;
        }
        ++len;
    }
    void push_back(const int& x) {
        node *p = new node(x);
        if (tail == nullptr) {
            head = tail = p;
        } else {
            tail->next = p;
            tail = p;
        }
        ++len;
    }
    void pop_front() {
        if (empty()) {
            cerr << "List is empty!" << endl;
            exit(EXIT_FAILURE);
        }
        node *p = head;
        head = head->next;
        delete p;
        --len;
        if (empty()) {
            tail = nullptr;
        }
    }
    void pop_back() {
        if (empty()) {
            cerr << "List is empty!" << endl;
            exit(EXIT_FAILURE);
        }
        if (head == tail) {
            delete tail;
            head = tail = nullptr;
            len = 0;
            return;
        }
        node *p = head;
        while (p->next != tail) {
            p = p->next;
        }
        delete tail;
        tail = p;
        tail->next = nullptr;
        --len;
    }
    void insert(int k, const int& x) {
        if (k < 0 || k > len) {
            cerr << "Invalid index!" << endl;
            exit(EXIT_FAILURE);
        }
        if (k == 0) {
            push_front(x);
            return;
        }
        if (k == len) {
            push_back(x);
            return;
        }
        node *p = head;
        for (int i = 0; i < k - 1; ++i) {
            p = p->next;
        }
        p->next = new node(x, p->next);
        ++len;
    }
    void erase(int k) {
        if (k < 0 || k >= len) {
            cerr << "Invalid index!" << endl;
            exit(EXIT_FAILURE);
        }
        if (k == 0) {
            pop_front();
            return;
        }
        if (k == len - 1) {
            pop_back();
            return;
        }
        node *p = head;
        for (int i = 0; i < k - 1; ++i) {
            p = p->next;
        }
        node *q = p->next;
        p->next = q->next;
        delete q;
        --len;
    }
    void print() const {
        node *p = head;
        while (p != nullptr) {
            cout << p->data << " ";
            p = p->next;
        }
        cout << endl;
    }
    void merge(link &B) {
        if (B.empty()) {
            return;
        }
        if (empty()) {
            head = B.head;
            tail = B.tail;
            len = B.len;
            B.head = B.tail = nullptr;
            B.len = 0;
            return;
        }
        tail->next = B.head;
        tail = B.tail;
        len += B.len;
        B.head = B.tail = nullptr;
        B.len = 0;
    }
};

在lab9_2.cpp中声明两个整型链表A和B，分别插入5个元素，然后把B中的元素加入A的尾部的程序如下：

#include <iostream>
#include "link.h"
using namespace std;

int main() {
    srand(time(0));
    link A, B;
    for (int i = 0; i < 5; ++i) {
        int x = rand() % 100 + 1;
        A.push_back(x);
    }
    for (int i = 0; i < 5; ++i) {
        int x = rand() % 100 + 1;
        B.push_back(x);
    }
    cout << "A: ";
    A.print();
    cout << "B: ";
    B.print();
    A.merge(B);
    cout << "After merging B into A:" << endl;
    cout << "A: ";
    A.print();
    cout << "B: ";
    B.print();
    return 0;
}

其中，srand(time(0))是为了每次运行程序时生成的随机数不同。在测试时，可以将其注释掉。