3292. Semi-prime H-numbers

23434836

POJ/3292/3292.cpp

@@ -0,0 +1,39 @@
+#include <iostream>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 1e6;
+
+int n, p, h_primes[N + 5], sum[N + 5];
+bool not_h_prime[N + 5], is_h_semi_prime[N + 5];
+
+int main() {
+    std::ios::sync_with_stdio(false);
+
+    for (int i = 5; i <= N; i += 4) {
+        if (!not_h_prime[i]) {
+            h_primes[++p] = i;
+            for (int j = i * 5; j <= N; j += i * 4) {
+                not_h_prime[j] = true;
+            }
+        }
+    }
+
+    for (int i = 1; i <= p; i++) {
+        for (int j = 1; j <= i && h_primes[i] * h_primes[j] <= N; j++) {
+            is_h_semi_prime[h_primes[i] * h_primes[j]] = true;
+        }
+    }
+
+    for (int i = 1; i <= N; i++) {
+        sum[i] = sum[i - 1] + is_h_semi_prime[i];
+    }
+
+    while (cin >> n, n) {
+        cout << n << ' ' << sum[n] << endl;
+    }
+
+    return 0;
+}