算法训练 P0505

P0505

#include<iostream>
#include<math.h>
using namespace std;
int a=0, b=0;
int main(){
    long long int n, ans=1, t;
    cin>>n;
    for(int i=1; i<=n; i++){
        t=i;
        while(t%2==0){
            a++;
            t/=2;
        }
        while(t%5==0){
            b++;
            t/=5;
        }
        if(t!=0)
            ans*=t;
        ans=ans%10;
    }
    if(a>b){
        switch((a-b)%4){
            case 0: ans=ans*6; break;
            case 1: ans=ans*2; break;
            case 2: ans=ans*4; break;
            case 3: ans=ans*8; break;
            default: break;
        }
    }else if(b>a)
        ans*=5;
    ans=ans%10;
    cout<<ans<<endl;
    return 0;
}