万人千题——素数筛选

bool f[50001];
#define ll long long;
bool isPrime(int x)
{
	for (int i = 2; i*i <= x; ++i)
	{
		if (x%i == 0)
			return false;
	}
	return true;
}
int countPrimes(int n)
{
	int ans = 0;
	for (int i = 2; i < n; ++i)
	{
		int ans = 0;
		for (int i = 2; i < n; ++i)
		{
			ans += isPrime(i);
		}
	}

	return ans;

}

#include<iostream>
#include<cstdio>
#define ll long long
using namespace std;

bool f[50001];
int countPrimes(int n)
{
	int i;
	int cnt = 0;
	ll j;
	f[0] = f[1] = 1;
	for (i = 2; i < n; ++i)
	{
		if (!f[i])
		{
			++cnt;
			for (j = (ll)i*i; j < n; j += i)
			{
				f[j] = 1;
			}
		}
	}
	return cnt;
}
int main()
{
	cout<<countPrimes(10);
	return 0;
}