题解 | #火车进站#

这道题还是比较难的, 我最先开始想的也是递归, 广度就是入栈和出栈, 但是做的事情不一样, 所以无从下笔. 鼓捣一阵无果后查看答案发现了两点, 广度是进栈和出栈, 但是返回到原函数需要还原, 入栈元素的下标用i表示, 深度和结束条件相关, 就是所有元素出栈完了, 出栈了arr.length()次的时候返回原函数, 结束递归

import java.util.*;

public class Main
{
    public static ArrayList<String> list = new ArrayList<>();
    public static StringBuilder sb = new StringBuilder();
    public static void main(String[] args)
    {
        Scanner sc = new Scanner(System.in);
        Stack<Integer> stack = new Stack<>();
        int n = sc.nextInt();
        int []arr = new int[n];
        for (int i = 0; i < n; i++)
        {
            arr[i] = sc.nextInt();
        }
        recursion(0,0,arr,stack);
        Collections.sort(list);
        for (String i: list)
        {
            System.out.println(i);
        }
    }
//1   2   3
    public static void recursion( int len, int i, int []arr ,Stack<Integer> stack  )
    {
        if(len == arr.length)
        {
            list.add(sb.toString());
            return;
        }
        //出栈
        if( !stack.isEmpty())
        {
            int k = stack.pop();
            sb.append(k+" ");
            recursion(len+1,i,arr,stack);
            stack.push(k);
            sb.delete(sb.length()-2, sb.length());

            //sb.deleteCharAt(sb.length()-1);
        }
        //入栈
        if(i< arr.length)
        {
            stack.push(arr[i]);
            recursion(len,i+1,arr,stack);
            stack.pop();
        }
    }
}