题解 | #火车进站#

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

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        // 注意 hasNext 和 hasNextLine 的区别
        while (in.hasNext()) {
            int num = in.nextInt();
            List<String> orders = new ArrayList<>();
            int[] trains = new int[num];
            for(int i=0 ; i<num ; ++i){
                trains[i] = in.nextInt();
            }
            dfs(orders, "", new Stack<Integer>(), trains, 0, 0);
            Collections.sort(orders);
            for(String order : orders){
                System.out.println(order);
            }


        }
    }

    public static void dfs(List<String> orders, String str,
                           Stack<Integer> stack, int[] trains, int pushNum, int popNum) {
        if(popNum == trains.length){
            orders.add(str);
        }

        if(pushNum < trains.length){ //如果栈没满，就先进栈，如果已经全部进栈了，则开启出栈模式
            stack.push(trains[pushNum]);
            dfs(orders, str, stack, trains, pushNum+1, popNum);
            stack.pop();  //恢复现场，恢复进栈前的状态
        }

        if(!stack.isEmpty()){ //如果当前栈有元素，则出去一个,然后再进栈和出栈，即中间进站的火车先出去，然后再执行后续的进站出站流程
            int x = stack.pop();
            dfs(orders, str+x+" ", stack, trains, pushNum, popNum+1);
            stack.push(x);  //恢复出栈前的状态
        }
    }
}