4、质数

AcWing 866. 试除法判定质数

AcWing 867. 分解质因数

868. 筛质数

#include <iostream>
#include <algorithm>

using namespace std;

bool is_prime(int x)
{
    if (x < 2) return false;
    for (int i = 2; i <= x / i; i ++ )
        if (x % i == 0)
            return false;
    return true;
}

int main()
{
    int n;
    cin >> n;

    while (n -- )
    {
        int x;
        cin >> x;
        if (is_prime(x)) puts("Yes");
        else puts("No");
    }

    return 0;
}

#include <iostream>
#include <algorithm>

using namespace std;

void divide(int x)
{
    for (int i = 2; i <= x / i; i ++ )
        if (x % i == 0)
        {
            int s = 0;
            while (x % i == 0) x /= i, s ++ ;
            cout << i << ' ' << s << endl;
        }
    if (x > 1) cout << x << ' ' << 1 << endl;
    cout << endl;
}

int main()
{
    int n;
    cin >> n;
    while (n -- )
    {
        int x;
        cin >> x;
        divide(x);
    }

    return 0;
}

#include <iostream>
#include <algorithm>

using namespace std;

const int N= 1000010;

int primes[N], cnt;
bool st[N];

void get_primes(int n)
{
    for (int i = 2; i <= n; i ++ )
    {
        if (st[i]) continue;
        primes[cnt ++ ] = i;
        for (int j = i + i; j <= n; j += i)
            st[j] = true;
    }
}

int main()
{
    int n;
    cin >> n;

    get_primes(n);

    cout << cnt << endl;

    return 0;
}