题解 | #火车站出站#

import java.util.*;

public class Main{

static List<String> l = new ArrayList<>(); //储存结果

public static void main(String[] args) {
    Scanner in = new Scanner(System.in);

    while (in.hasNext()) {
        l.clear(); //静态变量，每次先清空
        int nums = in.nextInt();
        int[] id = new int[nums];
        Stack<Integer> stack = new Stack<>();
        for (int i = 0; i < nums; i++) {
            id[i] = in.nextInt();
        }
        trainOut(id, 0, stack, "", 0);
       //对结果集排序
        Collections.sort(l); 
        for (String str : l) {
            System.out.println(str);
        }
    }
    in.close();
}

//i为入栈次数，n为出栈次数，str存储一趟结果

public static void trainOut(int[] id, int i, Stack<Integer> s, String str, int n) {
    if (n == id.length) {
        l.add(str); //如果所有火车均出栈则将当前结果保存
    }

    if (!s.empty()) { //栈非空时出栈
        int temp = s.pop();
        trainOut(id, i, s, str + temp + " ", n + 1);
        s.push(temp); //恢复现场
    }

    if (i < id.length) {
        s.push(id[i]);
        trainOut(id, i + 1, s, str, n);
        s.pop(); //恢复现场

    }
}

}