我们蒟蒻写暴力依旧徐徐渐进

直接暴力枚举拿到10pts。

#include <bits/stdc++.h>
using namespace std;
bool f(int x);
int main(){
    int l,r;
    int cnt = 0;
    
    cin >> l >> r;
    for(int i = l;i<=r;++i){
        if(f(i)){
            ++cnt;
        }
    }
    cout<<cnt<<endl;
}
bool f(int x){
    int c = 0;
    for(int i = 2;i<x;++i){
        if(x % i == 0){
            ++c;
        }
    }
    if(c!=0){return false;}
    else {return true;}
}

即使一顿优化也只能到40pts的水平。

#include <bits/stdc++.h>
using namespace std;
bool f(int x);
int main(){
    cin.tie(0)->sync_with_stdio(0);
    long long l,r;
    int cnt = 0;
    
    cin >> l >> r;
    for(int i = l;i<=r;++i){
        if(f(i)){
            ++cnt;
        }
    }
    cout<<cnt<<'\n';
}
bool f(int x){
    if (x <= 1) return 0;
    if (x == 2) return 1;
    if (x % 2 == 0) return 0;
    for(int i = 2;i<=sqrt(x);++i){
        if(x % i == 0){
            return 0;
        }
    }
    return 1;
}

所以直接套筛法草过去

#include <bits/stdc++.h>
using namespace std;
#define ll long long

void sieve(ll l, ll r) {
    vector<bool> p(r - l + 1, 1); 
    if (l == 1) p[0] = 0;
    for (ll i = 2; i * i <= r; ++i) {
        ll start = max(i * i, ((l + i - 1) / i) * i);
        for (ll j = start; j <= r; j += i) {
            p[j - l] = 0;
        }
    }
    int cnt = 0;
    for (ll i = l; i <= r; ++i) {
        if (p[i - l]) cnt++;
    }
    cout << cnt << '\n';
}
int main() {
    cin.tie(0)->sync_with_stdio(0);

    ll l, r;
    cin >> l >> r;
    sieve(l, r);
}