题解 | 阶乘末尾非零数字

#include<iostream>

using namespace std;

int main(){
    int n, t, c5=0, c2=0,ans=1;
    cin>>n;
    if (n==0||n==1) {
        cout << 1 << endl;
        return 0;
    }

    for(int i=1;i<=n;i++){
        t=i;
        while(t%5==0){
            c5++;
            t/=5;
        }
        while(t%2==0){
            c2++;
            t/=2;
        }
        ans*=t;
        ans%=10;
    }

    t=c2-c5;
    int n2[4]={6, 2, 4, 8};
    if(t!=0){
        ans*=n2[t%4];
        ans%=10;
    }
    cout<<ans;
    return 0;
}