AcWing 874. 筛法求欧拉函数
原题链接
简单
作者:
Drifter
,
2021-01-15 21:16:34
,
所有人可见
,
阅读 260
进阶 Day 1
#include <iostream>
#include <algorithm>
using namespace std;
const int N = 1e6 + 10;
typedef long long LL;
int n, cnt;
int phi[N], primes[N];
bool st[N];
LL get_eulers_sum(int n)
{
phi[1] = 1;
for(int i = 2; i <= n; i++)
{
if(!st[i])
{
primes[cnt++] = i;
phi[i] = i - 1;
}
for(int j = 0; primes[j] <= n / i; j++)
{
st[primes[j] * i] = true;
if(i % primes[j] == 0)
{
phi[primes[j] * i] = phi[i] * primes[j];
break;
}
phi[primes[j] * i] = phi[i] * (primes[j] - 1);
}
}
LL res = 0;
for(int i = 1; i <= n; i++) res += phi[i];
return res;
} //get_eulers_sum
int main(void)
{
scanf("%d", &n);
LL res = get_eulers_sum(n);
printf("%lld", res);
return 0;
}