素数筛法

高效找出2-n之间的所有素数。

#include <iostream>
#include <vector>
#include <cmath>
using namespace std;

int n = 1000000;
void PrimeSelect(vector<int> &prime, vector<bool> &mark) {
  for (int i = 2; i <= n; i++) {
    if (mark[i] == true) {
      continue;
    }
    prime.push_back(i);
    if (i > sqrt(n)) {
      continue;
    }
    for (int j = i * i; j <= n; j += i) {
      mark[j] = true;
    }
  }
}

int main() {
  vector<int> prime;
  vector<bool> mark(n + 1, false);
  PrimeSelect(prime, mark);
  for (int i = 0; i < prime.size(); i++) {
    cout << prime[i] << " ";
  }
  cout << endl;
  return 0;
}