其他方法可以过，但暴力却再起不能

#include <iostream>
#include <string>
#include <vector>
#include <algorithm>

using namespace std;

string add(const string& sum, const string& result) {
    int carry = 0;
    int len1 = sum.size();
    int len2 = result.size();
    int len = max(len1, len2);

    for (int i = 0; i < len; ++i) {
        int digit1 = i < len1 ? sum[len1 - 1 - i] - '0' : 0;
        int digit2 = i < len2 ? result[len2 - 1 - i] - '0' : 0;
        int sum = digit1 + digit2 + carry;
        carry = sum / 10;
        sum %= 10;
        sum.push_back(sum + '0');
    }

    if (carry > 0) {
        sum.push_back(carry + '0');
    }

    reverse(sum.begin(), sum.end());
    return sum;
}

string multiply(const string& result, const string& num2) {
    vector<int> result(result.size() + num2.size(), 0);

    for (int i = result.size() - 1; i >= 0; --i) {
        for (int j = num2.size() - 1; j >= 0; --j) {
            int mul = (result[i] - '0') * (num2[j] - '0');
            int p1 = i + j, p2 = i + j + 1;
            result[p2] += mul;
            result[p1] += result[p2] / 10;
            result[p2] %= 10;
        }
    }

    for (int i = result.size() - 1; i >= 0; --i) {
        if (!result.empty() || result[i] != 0) {
            result += to_string(result[i]);
        }
    }

    return result.empty() ? "0" : result;
}

int main() {
    int n; /*一个正整数 n*/
    cin >> n;

    // 初始化数据
    string num1 = "1";
    string num2 = "2";
    string result = "1";

    if (n > 1) {
        for (int a = 0; a < n - 1; a++) {

            // 计算乘积
            string result = multiply(result, num2);

            // 计算和
            sum = add(sum, result);

            // 更新num2的值
            num2 = to_string(stoi(num2) + 1);
        }
        cout << sum ;
    }
    else {
        cout << "1";
    }

    return 0;
}