参照牛客931628554号的代码，对变量命名做了一些优化。

import java.util.Scanner;
import java.util.*;


public class Main {
    static List<String> res = new ArrayList<>();   //存放结果，也就是所有可能的出站序列
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        while (in.hasNext()) {
            res.clear();
            int n = in.nextInt();//火车的数量
            int[] trains = new int[n];//存放火车编号
            Stack<Integer> station = new Stack<>();//用栈表示车站，只能先进后出
            for (int i = 0; i < n; i++) {
                trains[i] = in.nextInt();
            }
            trainOut(trains, 0, station, "", 0);
            Collections.sort(res); 
            for (String s : res) {
                System.out.println(s);
            }
        }
        in.close();
    }
    public static void trainOut(int[] trains, int in, Stack<Integer> station,
                                String res_temp, int out) {
        if (out == trains.length) {   //out表示已经出站的火车数量。当所有火车出站时，表示一个出站序列完成，将其添加到结果中
            res.add(res_temp);
        }
        if (!station.empty()) {  //当车站还有火车时
            int train = station.pop();  //出站一辆火车
            trainOut(trains, in, station, res_temp + train + " ", out + 1);//该出站火车添加到当前出站序列红，出站火车数量+1
            station.push(train);
        }
        if (in < trains.length) { //当还有火车未进站时
            station.push(trains[in]);//进站一辆火车
            trainOut(trains, in + 1, station, res_temp, out);//已进站火车数量+1
            station.pop();
        }
    }
}