Project Euler

A. Multiples of 3 and 5

大水题，计算出below n内是3，5倍数的总和

数据较大，只能用求和公式

#include <iostream>
#include <cstdio>
typedef long long ll;
using namespace std;

int main()
{
    int T;
    scanf("%d",&T);
    while(T--){
        int n;
        scanf("%d",&n);
        ll sum=0;
        ll p;
        p = (n-1)/3;
        sum += (1LL)*3*(p+1)*p/2;
        p = (n-1)/5;
        sum += (1LL)*5*(p+1)*p/2;
        p = (n-1)/15;
        sum -= (1LL)*15*(p+1)*p/2;
        cout<<sum<<endl;
    }
    //cout << "Hello world!" << endl;
    return 0;
}