'commit'

TangDou/AcWing/Combinations/889.cpp

@@ -0,0 +1,29 @@
+#include <bits/stdc++.h>
+
+using namespace std;
+typedef long long LL;
+const int p = 1e9 + 7;
+int n;
+
+int qmi(int a, int b) {
+    int res = 1;
+    while (b) {
+        if (b & 1) res = (LL)res * a % p;
+        b >>= 1;
+        a = (LL)a * a % p;
+    }
+    return res;
+}
+
+int main() {
+    cin >> n;
+    int res = 1;
+
+    // ① 分子,2n*(2n-1)*(2n-2)*...*(n+1)
+    for (int i = 2 * n; i > n; i--) res = (LL)res * i % p;
+    // ② 分母,inv(n+1)*inv(n)*inv(n-1)*...*inv(1) %p
+    for (int i = n + 1; i; i--) res = (LL)res * qmi(i, p - 2) % p;
+    // 输出
+    cout << res << endl;
+    return 0;
+}