题解 | 小红的双生排列

import java.math.BigDecimal;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        long n = in.nextLong();
        long sum = 0;
        if(n%2==1){
            long odd = n/2+1;
            long even = n/2;
            long sOdd = 1;
            for(long i = 1;i<=odd;i++){
                sOdd *=i;
                if(sOdd>1000000007){
                    sOdd = sOdd%1000000007;
                }
            }
            long sEven = 1;
            for(long i = 1;i<=even;i++){
                sEven *=i;
                if(sEven>1000000007){
                    sEven %= 1000000007;
                }
            }
            sum = (sEven*sOdd)%1000000007;
            System.out.println(sum);
        }else {
            long odd = n/2;
            long even = n/2;
            long sOdd = 1;
            for(long i = 1;i<=odd;i++){
                sOdd *=i;
                if(sOdd>1000000007){
                    sOdd = sOdd%1000000007;
                }
            }
            long sEven = sOdd;
            sum =  (sEven*sOdd*2)%1000000007;
            System.out.println(sum);
        }
    }
}