add: 220306 [cpp]

cpp/2203/220306.cpp

@@ -0,0 +1,25 @@
+#include <iostream>
+
+class Solution {
+private:
+	inline static const int MOD = 1000000007;
+public:
+	// OEIS A000680		a(n) = (2n)!/2^n.
+	static constexpr int countOrders(int n) {
+		if (n == 1)
+			return 1;
+		if (n == 2)
+			return 6;
+		unsigned long long ret = 6;
+		for (int i = 3; i <= n; ++i) {
+			ret = (ret * i) % MOD;
+			ret = (ret * ((i << 1) - 1)) % MOD;
+		}
+		return ret % MOD;
+	}
+};
+
+int main() {
+	std::cout << Solution::countOrders(5);
+	return 0;
+}

cpp/2203/CMakeLists.txt

@@ -3,4 +3,4 @@ PROJECT(2203)
 
 SET(CMAKE_CXX_STANDARD 23)
 
-ADD_EXECUTABLE(2203 220306-CN.cpp)
+ADD_EXECUTABLE(2203 220306.cpp)