题解 | 小红的排列构造①

import java.util.Scanner;

/**
 * @author zengxiangbao
 * @date 2025/10/11 14:19
 * @description BGN76 小红的排列构造①
 */
public class Main {
  	//1.对于n < 3的明显不存在排列
  	//2.对于n > 3 && n % 2 == 1的直接反过来输出就行了
  	//3.对于n > 3 && n % 2 == 0的从后往前依次将奇数对奇数， 偶数对偶数一定是一个可行的排列
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        if (n < 3) System.out.println(-1);
        else {
		  	
            boolean isOdd = n % 2 == 1, flag = false;
            int p1 = n - 1, p2 = n;
            while (p1 >= 1 || p2 >= 1) {
                System.out.print(isOdd ? (p2 >= 1 ? (flag ? " " : "") + p2 : ""): (p1 >= 1 ? (flag ? " " : "") + p1 : ""));
                if (!flag) flag = true;
                System.out.print(isOdd ? (p1 >= 1 ? " " + p1 : "") : (p2 >= 1 ? " " + p2 : ""));
                p1 -= 2;
                p2 -= 2;
            }
        }
    }
}