模拟与高精度 P1009 阶乘之和

题目

https://www.luogu.com.cn/problem/P1009

代码

#include<iostream>
#include<string>
#include<cstring>
#include<algorithm>
#include<cstdio>
#include<vector>
using namespace std;
#define max 2010

vector<int> mul(vector<int> a, int b)
{
    vector<int>p;
    int t = 0;
    for (int i = 0; i < a.size(); i++)
    {
        p.push_back((t + a[i] * b) % 10);
        t = (t + a[i] * b) / 10;
    }
    while (t != 0)
    {
        p.push_back(t % 10);
        t /= 10;
    }
    return p;
}
vector<int>add(vector<int>a, vector<int>b)
{
    vector<int>p,tmpa,tmpb;
    tmpa = a; tmpb = b;
    int aa = tmpa.size(), bb = tmpb.size();
    if (aa > bb)
        for (int i = 0; i < aa - bb;i++)tmpb.push_back(0);
    if (aa < bb)
        for (int i = 0; i < bb-aa; i++)tmpa.push_back(0);
    int t = 0;
    for (int i = 0; i < tmpa.size(); i++)
    {
        p.push_back((t + tmpa[i] + tmpb[i]) % 10);
        t = (t + tmpa[i] + tmpb[i]) / 10;
    }
    if (t != 0)
        p.push_back(t);
    return p;
}
int main()
{
    std::ios::sync_with_stdio(false);
    std::cin.tie(0);
    std::cout.tie(0);
    int t;
    cin >> t;
    vector<int>p,answer;
    p.push_back(1);
    answer.push_back(1);
    for (int i = 2; i <= t; i++)
    {
        p = mul(p, i);
        answer = add(answer, p);
    }
    bool ok = false;
    for (int i = answer.size() - 1; i >= 0; i--)
    {
        if (answer[i] != 0)ok = true;
        if (ok)cout << answer[i];
    }
    if (!ok)cout << 0;
}