From 5a0b6bc880cde939fc2fd07a9115ae4e9d0a8379 Mon Sep 17 00:00:00 2001
From: subcrip <contact@subc.rip>
Date: Sat, 30 Mar 2024 12:07:21 +0800
Subject: [PATCH] backup

---
 number/lpf.cc | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)
 create mode 100644 number/lpf.cc

diff --git a/number/lpf.cc b/number/lpf.cc
new file mode 100644
index 0000000..d5a788b
--- /dev/null
+++ b/number/lpf.cc
@@ -0,0 +1,19 @@
+constexpr int N = 1e7 + 10;
+
+int lpf[N];
+
+void era(int n) {
+    lpf[0] = lpf[1] = -1;
+    for (int i = 2; i <= n; ++i) lpf[i] = i;
+    for (int i = 2; i <= n; ++i) {
+        if (lpf[i] == i) {
+            if ((ll)i * i > n) continue;
+            for (int j = i * i; j <= n; j += i) {
+                if (lpf[j] == j) {
+                    lpf[j] = i;
+                }
+            }
+        }
+    }
+}
+