欧拉函数（线性筛）（超好Dong）

#include <bits/stdc++.h>

using namespace std;
const int maxn = 1e6;
bool vis[maxn];
int prime[maxn];
int phi[maxn];    

void init()
{
    memset(vis, false, sizeof(vis));
    phi[1] = 1;
    int cnt = 0;
    for(int i = 2; i < maxn; i ++)
    {
        if(!vis[i]){
            prime[cnt++] = i;
            phi[i] = i - 1;
        }
        for(int j = 0; j < cnt && i * prime[j] < maxn; j ++)
        {
            vis[i * prime[j]] = true;
            if(i % prime[j] == 0){
                phi[i*prime[j]] = phi[i]*prime[j];
                break;
            }
            else{
                phi[i*prime[j]] = phi[i]*phi[prime[j]];   // phi[i]*(prime[j]-1);
            }
        }
    }
}

int main()
{
    int n;
    cin >> n;
    init();
    cout << phi[n]<<endl;
}