半个小时看懂了大佬写的10行代码

#include 
     
      #include 
      
       #include 
       
        using namespace std;const int maxn = 40000;int f[40000];int n;int main(){    while(scanf("%d",&n)!=EOF)    {        memset(f,0,sizeof(f));        f[0] = 1;        for(int i = 2; i <= n; i++)        {            int c = 0;//低位来的进位            for(int j = 0; j < maxn; j++)            {                int s = f[j]*i+c;                f[j] = s%10;                c = s/10;            }        }        int flag = 0;        for(int i = maxn-1; i >= 0; i--)        {            if(f[i]!=0)            {                flag = i;                break;            }        }        for(int j = flag; j >= 0; j--)            printf("%d",f[j]);        printf("\n");    }    return 0;}