#1665. 【2022.10.19 联考】排列

https://sjzezoj.com/submission/61143

S2OJ/1665/1665.cpp

@@ -0,0 +1,48 @@
+#include <iostream>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 1e6 + 5;
+const int mod = 1e9 + 7;
+
+int n;
+long long f[N], ans;
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    cin >> n;
+
+    if (n == 1) {
+        cout << 1 << endl;
+
+        exit(0);
+    }
+
+    f[1] = 1;
+
+    for (int j = 1; j < n; j++) {
+        if (j <= 3) {
+            for (int i = j + 1; i < n; i++) {
+                f[i] = (f[i] + f[j]) % mod;
+            }
+        } else {
+            for (int k = -1; k <= 1; k++) {
+                for (int i = j + k; i < n; i += j) {
+                    if (j < i) f[i] = (f[i] + f[j]) % mod;
+                }
+            }
+        }
+    }
+
+    for (int i = 1; i < n; i++) {
+        ans = (ans + f[i]) % mod;
+    }
+
+    cout << ans * n % mod << endl;
+
+    return 0;
+}