AC 250, tried atcoder virtual contest, wasn't even half bad

atcoder/beginner_contests/250/d.cpp

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+#include <bits/stdc++.h>
+
+using namespace std;
+
+typedef unsigned long long ull;
+
+vector<ull> primes(ull maxPrime){
+    vector<bool> p(maxPrime + 1, true);
+
+    for(ull i = 2; i <= sqrt(maxPrime + 1); ++i){
+        if(p[i]){
+            for(ull j = i * 2; j < maxPrime + 1; j += i){
+                p[j] = false;
+            }
+        }
+    }
+
+    vector<ull> result;
+    for(ull i = 2; i < maxPrime + 1; ++i){
+        if(p[i]){
+            result.push_back(i);
+        }
+    }
+
+    return result;
+}
+
+
+int main(){
+    ios::sync_with_stdio(0);
+    cin.tie(0);
+
+    ull n;
+    cin >> n;
+
+    ull topPrime = ceil(cbrt(ceil(n / 2)));
+    auto ps = primes(topPrime);
+
+    ull result = 0;
+
+    if(ps.size() < 2){
+        cout << result << endl;
+        cout << flush;
+        return 0;
+    }
+
+    for(size_t i = 1; i < ps.size(); ++i){
+        ull q = ps[i];
+
+        for(size_t j = 0; j < i; ++j){
+            ull p = ps[j];
+            ull k = p * q * q * q;
+
+            if(k > n){
+                break;
+            }
+            result++;
+        }
+    }
+
+    cout << result << endl;
+
+    cout << flush;
+
+    return 0;
+}