题目链接:https://www.luogu.org/problemnew/show/P1009
题目大意:求
#include<iostream>
#include<string>
#include<cstring>
#include<algorithm>
using namespace std;
const int maxn = 100000 + 10;
int a[maxn],an[maxn],bn[maxn];
int n;
string fac(int n){
if(n == 0) return "1";
memset(a, 0, sizeof a);
int tmp = n, len = 0;
while(tmp) a[len++] = tmp % 10, tmp /= 10; //将n放进数组中
for(int i = n-1; i >= 2; i--){
int c = 0; //进位
for(int j = 0;j < len; j++) a[j] = a[j] * i + c, c = a[j] / 10, a[j] %= 10;
while(c) a[len++] = c % 10, c /= 10;
}
string res="";
for(int i = 0; i < len; i++)
res+=a[len-i-1] +'0';
return res;
}
string add(string a,string b){
memset(an, 0, sizeof an),memset(bn, 0,sizeof bn);
int lena = a.length(),lenb = b.length();
for(int i = 0;i < lena; i++) an[lena-i-1] = a[i] - '0';
for(int i = 0;i < lenb; i++) bn[lenb-i-1] = b[i] - '0';
int len = max(lena,lenb);
for(int i = 0;i < len; i++){
an[i]+=bn[i]; an[i+1] +=an[i]/10; an[i] %= 10;
}
if(an[len]) len++;
string res="";
for(int i=0;i<len;i++){
res+=an[len-1-i]+'0';
}
return res;
}
int main()
{
string res="0";
cin>>n;
while(n){
res=add(res,fac(n));
n--;
}
cout<<res<<endl;
return 0;
}