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();
        BigDecimal sum = new BigDecimal(0);
        if (n % 2 == 1) {
            long odd = n / 2 + 1;
            long even = n / 2;
            BigDecimal sOdd = new BigDecimal(1);
            for (long i = 1; i <= odd; i++) {
                sOdd = sOdd.multiply(new BigDecimal(i));
                if (sOdd.compareTo(new BigDecimal(1000000007)) == 1) {
                    sOdd = sOdd.remainder(new BigDecimal(1000000007));
                }
            }
            BigDecimal sEven = new BigDecimal(1);
            for (long i = 1; i <= even; i++) {
                sEven = sEven.multiply(new BigDecimal(i));
                if (sEven.compareTo(new BigDecimal(1000000007)) == 1) {
                    sEven = sEven.remainder(new BigDecimal(1000000007));
                }
            }
            sum = (sEven.multiply(sOdd)).remainder(new BigDecimal(1000000007));
            System.out.println(sum);
        } else {
            long odd = n / 2;
            long even = n / 2;
            BigDecimal sOdd = new BigDecimal(1);
            for (long i = 1; i <= odd; i++) {
                sOdd = sOdd.multiply(new BigDecimal(i));
                if (sOdd.compareTo(new BigDecimal(1000000007)) == 1) {
                    sOdd = sOdd.remainder(new BigDecimal(1000000007));
                }
            }
            BigDecimal sEven = sOdd;
            sum = (sEven.multiply(sOdd).multiply(new BigDecimal(2))).remainder(
                      new BigDecimal(1000000007));
            System.out.println(sum);
        }
    }
}