2020-08-17 阶乘之和

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

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <sstream>
#include <algorithm>
#include <cstring>
using namespace std;

long long qmi(int m, int k)
{
    int res = 1, t = m;
    while (k)
    {
        if (k&1) res = res * t;
        t = t * t;
        k >>= 1;
    }
    return res;
}

int read(){
    int x = 0,f = 1;
    char c = getchar();
    while (c<'0'||c>'9') {
        if (c=='-') {
            f = -1;
        }
        c = getchar();
    }
    while (c>='0'&&c<='9') {
        x = x*10+c-'0';
        c = getchar();
    }
    return x*f;
}

struct fantastic     //嗯，开始重载了
{
    int len,s[9999];
    fantastic()
    {
        memset(s,0,sizeof(s));
        len=1;
    }
    fantastic operator=(const char*num)
    {
        len=strlen(num);
        for(int i=0;i<len;++i)
            s[i]=num[len-i-1]-'0';
        return *this;
    }
    fantastic operator=(const int num)
    {
        char a[9999];
        sprintf(a,"%d",num);
        *this=a;
        return *this;
    }
    fantastic (const int num)
    {
        *this=num;
    }
    fantastic (const char * num)
    {
        *this=num;
    }
    fantastic operator+(const fantastic &a)   //这里在重载 “+” 的运算
    {
        fantastic c;
        c.len=max(len,a.len)+1;                //这里就是我们熟悉的竖式模拟了
        for(int i=0,x=0;i<c.len;++i)
        {
            c.s[i]=s[i]+a.s[i]+x;
            x=c.s[i]/10;
            c.s[i]=c.s[i]%10;
        }
        if(c.s[c.len-1]==0)
            --c.len;
        return c;
    }
    fantastic operator * (const fantastic &x)           //然后再来波 “*” 的运算
    {
        fantastic c;
        c.len=len+x.len;                 //又是我们熟悉的竖式模拟
        for(int i=0;i<len;++i)
            for(int j=0;j<x.len;++j)
            {
                c.s[i+j]+=s[i]*x.s[j];
                c.s[i+j+1]+=c.s[i+j]/10;
                c.s[i+j]%=10;
            }
        if(c.s[c.len-1]==0)
            --c.len;
        return c;
    }
};
ostream& operator<<(ostream &out,const fantastic& x)   //重载一下输出
{
    for(int i=x.len-1;i>=0;--i)
        cout<<x.s[i];
    return out;
}
istream& operator>>(istream &in,fantastic &x)       //重载一下输入
{
    char num[9999];
    in>>num;
    x=num;
    return in;
}

int main()         //然后就可以愉快的开始主程序啦
{
    int n;
    fantastic ans=0,num=1;
    cin>>n;
    for(int i=1;i<=n;i++)
    {
        num=num*i;
        ans=ans+num;
    }
    cout<<ans<<endl;
}                    /*
3
============
89
*/