求欧拉函数

筛法+试除
推荐视频：筛法求欧拉函数

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#include<bits/stdc++.h>
using namespace std;
#define LL long long
const int N = 1e8 + 10;
int p[N], cnt,phi[N];
bool vis[N];
void get_phi(int n) {//线性筛
	phi[1] = 1;
	for (int i = 2; i <= n; i++) {
		if (!vis[i]) {
			p[++cnt] = i;
			phi[i] = i - 1;
		}
		for (int j = 1; 1LL * i * p[j]<=n; j++) {
			int m = i * p[j];
			vis[m]=1;
			if (i % p[j] == 0) {
				phi[m] = p[j] * phi[i];
				break;
			}
			else {
				phi[m] = (p[j] - 1) * phi[i];
			}
		}
	}

}
//根据欧拉函数性质，只与质因子数量有关，与其次数无关
int phi_(int n) {//试除法
	int res = n;
	for (int i = 2; i * i <= n; i++) {
		if (n % i == 0) {
			res = res / i * (i - 1);
		}
		while (n % i == 0) n /= i;
	}
	if (n > 1) res = res / n * (n - 1);
	return res;
}
int main() {
	int n;
	cin >> n;
	phi_(n);
	get_phi(n);
	return 0;
}