题解 | 小红的双生排列

import java.util.Scanner;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        // 注意 hasNext 和 hasNextLine 的区别
        final long MOD = 1000000007;
        int count = in.nextInt();

        int odds  = (count % 2 == 1) ? (count  /2 + 1) : count/2;
        int event = count /2 ;

        long allSize = 1;
        for (int i = odds; i > 0; i--) {
            allSize = allSize * i % MOD;
        }

        for (int i = event; i > 0; i--) {
            allSize = allSize * i % MOD;
        }

        if(count % 2 == 0) {
            allSize = allSize * 2 % MOD;
        }
        System.out.println(allSize);
    }
}