使用封装好的大整数模板

#include<bits/stdc++.h>
using namespace std;
struct BigIntTiny
{
    int sign;
    std::vector<int> v;

    BigIntTiny() : sign(1) {}
    BigIntTiny(const std::string &s) { *this = s; }
    BigIntTiny(int v)
    {
        char buf[21];
        sprintf(buf, "%d", v);
        *this = buf;
    }
    void zip(int unzip)
    {
        if (unzip == 0)
        {
            for (int i = 0; i < (int)v.size(); i++)
                v[i] = get_pos(i * 4) + get_pos(i * 4 + 1) * 10 + get_pos(i * 4 + 2) * 100 + get_pos(i * 4 + 3) * 1000;
        }
        else
            for (int i = (v.resize(v.size() * 4), (int)v.size() - 1), a; i >= 0; i--)
                a = (i % 4 >= 2) ? v[i / 4] / 100 : v[i / 4] % 100, v[i] = (i & 1) ? a / 10 : a % 10;
        setsign(1, 1);
    }
    int get_pos(unsigned pos) const { return pos >= v.size() ? 0 : v[pos]; }
    BigIntTiny &setsign(int newsign, int rev)
    {
        for (int i = (int)v.size() - 1; i > 0 && v[i] == 0; i--)
            v.erase(v.begin() + i);
        sign = (v.size() == 0 || (v.size() == 1 && v[0] == 0)) ? 1 : (rev ? newsign * sign : newsign);
        return *this;
    }
    std::string to_str() const
    {
        BigIntTiny b = *this;
        std::string s;
        for (int i = (b.zip(1), 0); i < (int)b.v.size(); ++i)
            s += char(*(b.v.rbegin() + i) + '0');
        return (sign < 0 ? "-" : "") + (s.empty() ? std::string("0") : s);
    }
    bool absless(const BigIntTiny &b) const
    {
        if (v.size() != b.v.size())
            return v.size() < b.v.size();
        for (int i = (int)v.size() - 1; i >= 0; i--)
            if (v[i] != b.v[i])
                return v[i] < b.v[i];
        return false;
    }
    BigIntTiny operator-() const
    {
        BigIntTiny c = *this;
        c.sign = (v.size() > 1 || v[0]) ? -c.sign : 1;
        return c;
    }
    BigIntTiny &operator=(const std::string &s)
    {
        if (s[0] == '-')
            *this = s.substr(1);
        else
        {
            for (int i = (v.clear(), 0); i < (int)s.size(); ++i)
                v.push_back(*(s.rbegin() + i) - '0');
            zip(0);
        }
        return setsign(s[0] == '-' ? -1 : 1, sign = 1);
    }
    bool operator<(const BigIntTiny &b) const
    {
        return sign != b.sign ? sign < b.sign : (sign == 1 ? absless(b) : b.absless(*this));
    }
    bool operator==(const BigIntTiny &b) const { return v == b.v && sign == b.sign; }
    BigIntTiny &operator+=(const BigIntTiny &b)
    {
        if (sign != b.sign)
            return *this = (*this) - -b;
        v.resize(std::max(v.size(), b.v.size()) + 1);
        for (int i = 0, carry = 0; i < (int)b.v.size() || carry; i++)
        {
            carry += v[i] + b.get_pos(i);
            v[i] = carry % 10000, carry /= 10000;
        }
        return setsign(sign, 0);
    }
    BigIntTiny operator+(const BigIntTiny &b) const
    {
        BigIntTiny c = *this;
        return c += b;
    }
    void add_mul(const BigIntTiny &b, int mul)
    {
        v.resize(std::max(v.size(), b.v.size()) + 2);
        for (int i = 0, carry = 0; i < (int)b.v.size() || carry; i++)
        {
            carry += v[i] + b.get_pos(i) * mul;
            v[i] = carry % 10000, carry /= 10000;
        }
    }
    BigIntTiny operator-(const BigIntTiny &b) const
    {
        if (sign != b.sign)
            return (*this) + -b;
        if (absless(b))
            return -(b - *this);
        BigIntTiny c;
        for (int i = 0, borrow = 0; i < (int)v.size(); i++)
        {
            borrow += v[i] - b.get_pos(i);
            c.v.push_back(borrow);
            c.v.back() -= 10000 * (borrow >>= 31);
        }
        return c.setsign(sign, 0);
    }
    BigIntTiny operator*(const BigIntTiny &b) const
    {
        if (b < *this)
            return b * *this;
        BigIntTiny c, d = b;
        for (int i = 0; i < (int)v.size(); i++, d.v.insert(d.v.begin(), 0))
            c.add_mul(d, v[i]);
        return c.setsign(sign * b.sign, 0);
    }
    BigIntTiny operator/(const BigIntTiny &b) const
    {
        BigIntTiny c, d;
        d.v.resize(v.size());
        double db = 1.0 / (b.v.back() + (b.get_pos((unsigned)b.v.size() - 2) / 1e4) +
                           (b.get_pos((unsigned)b.v.size() - 3) + 1) / 1e8);
        for (int i = (int)v.size() - 1; i >= 0; i--)
        {
            c.v.insert(c.v.begin(), v[i]);
            int m = (int)((c.get_pos((int)b.v.size()) * 10000 + c.get_pos((int)b.v.size() - 1)) * db);
            c = c - b * m, d.v[i] += m;
            while (!(c < b))
                c = c - b, d.v[i] += 1;
        }
        return d.setsign(sign * b.sign, 0);
    }
    BigIntTiny operator%(const BigIntTiny &b) const { return *this - *this / b * b; }
    bool operator>(const BigIntTiny &b) const { return b < *this; }
    bool operator<=(const BigIntTiny &b) const { return !(b < *this); }
    bool operator>=(const BigIntTiny &b) const { return !(*this < b); }
    bool operator!=(const BigIntTiny &b) const { return !(*this == b); }
};

int main(){
	int n;
	cin >> n;
	BigIntTiny sum = 0;
	BigIntTiny now = 1;
	for(int i = 1; i <= n; i++)
	{
		now = now * i;
		sum += now;
	}
	cout << sum.to_str();
	return 0;
}

#include <bits/stdc++.h>
using namespace std;
const int LEN = 50 * 2 + 50;
int n;
int aLen, sLen;
int a[LEN + 5], s[LEN + 5];
int main()
{
    cin >> n;

    // 构建高精度的 0! 以及高精度的 0
    aLen = 1;
    a[0] = 1;
    sLen = 1;
    s[0] = 0;

    // n 次，每次算出 i!，然后累加进 s
    for (int i = 1; i <= n; i++)
    {
        // 【计算 i!】
        for (int j = 0; j < aLen; j++)
            a[j] *= i;
        for (int j = 0; j < aLen - 1; j++)
        {
            a[j + 1] += a[j] / 10;
            a[j] %= 10;
        }
        while (a[aLen - 1] > 9)
        {
            a[aLen] = a[aLen - 1] / 10;
            a[aLen - 1] %= 10;
            aLen++;
        }
        // 【s += i!】
        sLen = max(sLen, aLen);
        for (int i = 0; i < sLen; i++)
            s[i] += a[i];
        for (int j = 0; j < sLen - 1; j++)
        {
            s[j + 1] += s[j] / 10;
            s[j] %= 10;
        }
        while (s[aLen - 1] > 9)
        {
            s[sLen] = a[sLen - 1] / 10;
            s[sLen - 1] %= 10;
            sLen++;
        }
    }

    // 输出
    for (int i = sLen - 1; i >= 0; i--)
        cout << s[i];
    return 0;
}