874. 筛法求欧拉函数

https://www.acwing.com/problem/content/submission/code_detail/7814036/

AcWing/874/874.cpp

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+#include <bits/stdc++.h>
+
+using namespace std;
+
+int n, p, primes[1000005], phi[1000005];
+bool vis[1000005];
+long long ans;
+
+void get_eulers(int n) {
+    phi[1] = 1;
+    for (int i = 2; i <= n; i++) {
+        if (!vis[i]) {
+            primes[p++] = i;
+            phi[i] = i - 1;
+        }
+        for (int j = 0; primes[j] * i <= n; j++) {
+            vis[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);
+        }
+    }
+}
+
+int main() {
+    cin >> n;
+    get_eulers(n);
+    for (int i = 1; i <= n; i++) {
+        ans += phi[i];
+    }
+    cout << ans << endl;
+    return 0;
+}