python/TangDou/LuoGuBook/P3601_Prepare_1.cpp

#include <bits/stdc++.h>

using namespace std;
const int INF = 0x3f3f3f3f;

typedef long long LL;
LL l = 1, r = 2000000;

const int N = 1e7 + 10;
//欧拉筛[线性筛法]
int primes[N], cnt;     // primes[]存储所有素数
bool st[N];             // st[x]存储x是否被筛掉
void get_primes(int n) {
    for (int i = 2; i <= n; i++) {
        if (!st[i]) primes[cnt++] = i;
        for (int j = 0; primes[j] <= n / i; j++) {
            st[primes[j] * i] = true;
            if (i % primes[j] == 0) break;
        }
    }
}

vector<int> v[N];

int main() {
    /**=====================================================***/
    //计时开始
    clock_t startTime= clock();
    /**=====================================================***/

    //1、筛出小于sqrt(r)的所有质数
    get_primes(sqrt(r));

    //2、遍历每个数字，求质因子
    for (LL i = l; i <= r; i++) {
        int t = i;
        for (int j = 0; j < cnt && primes[j] * primes[j] <= i; j++) {
            if (i % primes[j] == 0) {
                v[i].push_back(primes[j]);
                //除干净这个质数因子
                while (t % primes[j] == 0) t /= primes[j];
            }
        }
        if (t > 1) v[i].push_back(t);
    }

    //输出质数因子有哪些
    for (int i = l; i <= r; i++) {
        cout << i << ":       ";
        for (auto c:v[i]) cout << c << " ";
        cout << endl;
    }
    cout << endl;

    /**************************************************************/
    //计时结束
    clock_t endTime = clock();
    cout << "The run time is: " <<(double)(endTime - startTime) / CLOCKS_PER_SEC << "s" << endl;
    /**=====================================================***/
    //The run time is: 4.273s
    return 0;
}