筛法求约数个数

#include<bits/stdc++.h>
using namespace std;
#define LL long long
const int N = 1e8 + 10;
int p[N], cnt;
int d[N];//d[i]记录i的约数的个数
int a[N];//a[i]记录i的最小质因子的次数
bool vis[N];
void get_d(int n) {//筛法求约数的个数
	d[1] = 1;
	for (int i = 2; i * i <= n; i++) {
		if (!vis[i]) {
			p[++cnt] = i;
			a[i] = 1; d[i] = 2;
		}
		for (int j = 1; 1LL * i * p[j]<=n; j++) {
			int m = i * p[j];
			vis[m]=1;
			if (i % p[j] == 0) {
				a[m] = a[i] + 1;
				d[m] = d[i] / a[m] * (a[m]+1);
				break;
			}
			else {//若i不能被p[j]整除
				a[m]=1;
				d[m] = d[m] * 2;
			}
		}
	}

}

int main() {
	int n;
	cin >> n;
	get_d(n);
	return 0;
}