P1835 素数密度

R67087367

Luogu/P1835/P1835.cpp

@@ -0,0 +1,36 @@
+#include <cstring>
+#include <iostream>
+
+using std::cin;
+using std::cout;
+using std::endl;
+
+const int N = 1000005;
+
+long long l, r, cnt, primes[N], ans;
+bool vis[N];
+
+int main() {
+    for (int i = 2; i <= 1000000; i++) {
+        if (!vis[i]) {
+            primes[++cnt] = i;
+            for (int j = i; j <= 1000000; j += i) {
+                vis[j] = true;
+            }
+        }
+    }
+    cin >> l >> r;
+    memset(vis, 0x00, sizeof(vis));
+    l = std::max(l, 2ll);
+    for (int i = 1; i <= cnt; i++) {
+        long long p = primes[i];
+        for (long long j = std::max((l + p - 1) / p * p, p * 2); j <= r; j += p) {
+            vis[j - l + 1] = true;
+        }
+    }
+    for (int i = 1; i <= r - l + 1; i++) {
+        if (!vis[i]) ans++;
+    }
+    cout << ans << endl;
+    return 0;
+}