火车出站序列：
思路：
三个变量：待进站火车个数，待出站火车个数，已出站火车个数

import com.sun.xml.internal.bind.v2.schemagen.xmlschema.List;import java.util.ArrayList;import java.util.Scanner;import java.util.Stack;import java.util.TreeSet;import java.util.Set;public class Train { public static boolean isLegal(int[] in,int[] out,int n){ int i=0; int j=0; Stack<Integer> stack=new Stack<>(); while (i<n){ if(in[i]==out[j]){ i++; j++; } else{ if(stack.empty()){ stack.push(in[i]); i++; } else { int top=stack.peek(); if(top==out[j]){ j++; stack.pop(); } else{ if(i<n){ stack.push(in[i]); i++; } } } } } while(!stack.empty()&&j<n){ int top=stack.pop(); if(top==out[j]){ j++; } else { return false; } } return true; }public static void swap(int[] arr,int i,int j){ int tmp=arr[i]; arr[i]=arr[j]; arr[j]=tmp;} public static void Permutation(int []arr,int start,int N, ArrayList<int[]> result){ if(start==N){ return; } if(start==N-1){ int [] str=arr.clone();//数组拷贝 result.add(str); return; } for(int i=start;i<N;i++){ swap(arr,i,start); Permutation(arr,start+1,N,result); swap(arr,i,start); } } public static void main(String[] args) { Scanner in=new Scanner(System.in); while(in.hasNext()) {//有多组测试用例 int N = in.nextInt();//[1,9] int[] arr = new int[N]; for (int i = 0; i < N; i++) { arr[i] = in.nextInt(); } ArrayList<int[]> result = new ArrayList<>();//很多组， int start = 0; Permutation(arr, start, N, result); Set<String> set = new TreeSet<String>(); for (int[] out : result) {//遍历result if (isLegal(arr, out, N)) { StringBuffer sb = new StringBuffer(); for (int i = 0; i < N - 1; i++) { sb.append(out[i] + " "); } sb.append(out[N - 1]); set.add(sb.toString()); } for (String s : set) {//遍历 里是string的set System.out.println(s); } } } }}