C++的无符号整数类型表示范围有限，能表示的十进制无符号整数不超过20位。编写程序，实现超过20位的无符号整数的加法和乘法运算。要求程序读入文本文件input.txt，计算其中的表达式的运算结果，并将结果输出到文本文件output.txt中。例如： 输入文件input.txt的内容 123456789012345678901234567890 + 123456789012345678901234567890 74795969103554554996215276693934490847811844274620 * 51 输出文件output.txt的内容 ( 123456789012345678901234567890 + 123456789012345678901234567890 = 246913578024691357802469135780 ) ( 74795969103554554996215276693934490847811844274620 * 51 = 3814594424281282304806979111390659033238404058005620 )

以下是C++的实现代码：

#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include <algorithm>
using namespace std;

vector<int> add(vector<int>& a, vector<int>& b) {
    vector<int> res;
    int carry = 0;
    int i = a.size() - 1, j = b.size() - 1;
    while (i >= 0 || j >= 0) {
        int sum = carry;
        if (i >= 0) sum += a[i--];
        if (j >= 0) sum += b[j--];
        res.push_back(sum % 10);
        carry = sum / 10;
    }
    if (carry) res.push_back(carry);
    reverse(res.begin(), res.end());
    return res;
}

vector<int> multiply(vector<int>& a, int b) {
    vector<int> res;
    int carry = 0;
    for (int i = a.size() - 1; i >= 0; i--) {
        int prod = a[i] * b + carry;
        res.push_back(prod % 10);
        carry = prod / 10;
    }
    if (carry) res.push_back(carry);
    reverse(res.begin(), res.end());
    return res;
}

int main() {
    ifstream fin("input.txt");
    ofstream fout("output.txt");

    string s1, s2;
    fin >> s1 >> s2;

    vector<int> a, b;
    for (int i = s1.size() - 1; i >= 0; i--) a.push_back(s1[i] - '0');
    for (int i = s2.size() - 1; i >= 0; i--) b.push_back(s2[i] - '0');

    vector<int> res_add = add(a, b);
    vector<int> res_mul = multiply(a, stoi(s2));

    fout << "(" << s1 << " + " << s2 << " = ";
    for (int i = 0; i < res_add.size(); i++) fout << res_add[i];
    fout << ")" << endl;

    fout << "(" << s1 << " * " << s2 << " = ";
    for (int i = 0; i < res_mul.size(); i++) fout << res_mul[i];
    fout << ")" << endl;

    fin.close();
    fout.close();
    return 0;
}

程序首先读入两个字符串s1和s2，分别表示要相加和相乘的两个数。然后将它们转化为vector<int>类型，方便进行加法和乘法操作。接下来调用add和multiply函数进行相应的操作，最后将结果输出到文件output.txt中。