题解 | #火车进站#

import java.util.ArrayList;
import java.util.Collections;
import java.util.Scanner;
import java.util.Stack;

public class Main {
    private static ArrayList<String> result = new ArrayList<>();

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

        while(in.hasNext()){
            solution(in);
        }
    }

    /**
     * 栈
     * @param in
     */
    private static void solution(Scanner in){
        int num = in.nextInt();
        int[] trains = new int[num];

        // 列车编号
        for(int i=0; i<num; i++){
            trains[i] = in.nextInt();
        }

        result = new ArrayList<>();

        // 火车站
        Stack<Integer> station = new Stack<>();

        // 发车
        depart(trains, station, 0, 0, "");

        Collections.sort(result);

        for(String seq: result){
            System.out.println(seq);
        }
    }

    /**
     * 递归
     * @param trains    列车初始编号序列
     * @param station   火车站
     * @param inNum     驶入火车站列车数
     * @param outNum    驶出火车站列车数
     * @param seqStr    驶出火车站列车序列
     */
    private static void depart(int[] trains, Stack<Integer> station, int inNum, int outNum, String seqStr){
        // 列车全部驶出
        if(outNum == trains.length){
            result.add(seqStr);
            return;
        }

        if(!station.isEmpty()){
            // 驶出
            Integer seq = station.pop();
            depart(trains, station, inNum, outNum+1, seqStr+seq+" ");
            // 恢复
            station.push(seq);
        }

        if(inNum < trains.length){
            // 驶入
            station.push(trains[inNum]);
            depart(trains, station, inNum+1, outNum, seqStr);
            // 恢复
            station.pop();
        }
    }
}