HDU-2824-The Euler function(欧拉函数)

• 欧拉筛

  Accepted

  2824

  343MS

  24856K

  546 B

  G++

  #include "bits/stdc++.h"
  using namespace std;
  typedef long long LL;
  const int MAXN = 3000005;
  LL euler[MAXN];
  void init() {
      for (int i = 2; i < MAXN; i++) {
          euler[i] = i;
      }
      for (int i = 2; i < MAXN; i++) {
          if (euler[i] == i) {
              for (int j = i; j < MAXN; j += i) {
                  euler[j] = euler[j] / i * (i - 1);
              }
          }
      }
      for (int i = 2; i < MAXN; i++) {
          euler[i] += euler[i - 1];
      }
  }
  int main() {
      init();
      int a, b;
      while (~scanf("%d%d", &a, &b)) {
          printf("%lld\n", euler[b] - euler[a - 1]);
      }
      return 0;
  } 

  只要知道素数筛和欧拉函数看这个代码就很轻松啦。就是模拟素数筛

• 欧拉函数(求φ(n))

  int euler(int n) {
      int ans = n;
      for (int i = 2; i * i <= n; i++) {
          if (n % i == 0) {
              ans = ans / i * (i - 1);
              while (n % i == 0) {
                  n /= i;
              }
          }    
      }
      if (n != 1) {
          ans = ans / n * (n - 1);
      }
      return ans;
  }

  附一个欧拉函数求法

HDU-2824-The Euler function(欧拉函数)

原文：https://www.cnblogs.com/Angel-Demon/p/10371878.html