Java递归兑现已知前序中序求后序

Java递归实现已知前序中序求后序
import java.util.*;
public class Main{
public static void main(String[] args) throws Exception{
Scanner in = new Scanner(System.in);
String pre,mid;
while(in.hasNext()){
pre = in.next();
mid = in.next();
System.out.println(PosOrder(pre,mid));
}
}

private static String PosOrder(String pre, String mid) throws Exception{
if(pre.length() == 1)
return pre;
else if(pre.length() == 0)
return "";
int m = mid.indexOf(pre.charAt(0));
return PosOrder(pre.substring(1,m+1 ),mid.substring(0, m)) + PosOrder(pre.substring(m+1),mid.substring(m+1)) + pre.charAt(0);


}
}