题解 | #火车进站# 相对简单
火车进站
http://www.nowcoder.com/practice/97ba57c35e9f4749826dc3befaeae109
import java.util.*;
public class Main{
public static TreeSet<String> res = new TreeSet<>();
public static void main(String[] args){
Scanner sc = new Scanner(System.in);
int N = Integer.valueOf(sc.nextLine());
String input = sc.nextLine();
String[] vals = input.split(" ");
LinkedList<String> ll = new LinkedList<String>();
LinkedList<String> temp = new LinkedList<String>();
int i=0,j=0;
traceBack(vals,ll,temp,i,j);
printResult();
}
//train表示火车站中的火车，in表示进站的计数，out表示出站的计数，ll表示出站的序列
public static void traceBack(String[] vals,LinkedList<String> ll,LinkedList<String> train,int in,int out){
int len = vals.length;
if( in == len && out == len){
StringBuffer sb = new StringBuffer();
for(int i=0;i<len;i++){
if(i > 0){
sb.append(" ");
}
sb.append(ll.get(i));
}
res.add(sb.toString());
return;
}
if( in != len){
train.add(vals[in]);
traceBack(vals,ll,train,in+1,out);
train.removeLast();
}
if( !train.isEmpty()){
String mid = train.removeLast();
ll.add(mid);
traceBack(vals,ll,train,in,out+1);
ll.removeLast();
train.add(mid);
}
}
public static void printResult(){
for(String x: res){
System.out.println(x);
}
}
} 
