请求添加题解

#include <iostream>
#include <cstring>
#include <cmath>

bool is_prime[50005];
int prime[50005],cnt = 0,ans = 0,l,r,sqrtR;

bool isPrime(int x){
    
    if (x < 2) return false;
    if (x == 2) return true;
    for (int i = 1;i <= cnt && prime[i] * prime[i] <= x;i++)//只用素数判断
        if (x % prime[i] == 0)
            return false;
    return true;
    
}

int main(){
    
    std::cin.tie(0)->sync_with_stdio(0);
    
    std::cin >> l >> r;
    sqrtR = ceil(sqrt(r));
    memset(is_prime,true,sizeof(bool) * sqrtR);
    is_prime[0] = is_prime[1] = false;
    for (int i = 2;i <= sqrtR;i++){//欧拉筛
        
        if (is_prime[i]) prime[++cnt] = i;
        for (int j = 1;j <= cnt && i * prime[j] <= sqrtR;j++){
            
            is_prime[i * prime[j]] = false;
            if (i % prime[j] == 0) break;
            
        }
        
    }
    for (long i = l;i <= r;i++)
        if (isPrime(i))
            ans++;
    std::cout << ans << '\n';
    
    return 0;
}