java创建递归二叉树，输出根数据时出现空指针异常

Question

代码如下：
import java.io.File;
import java.io.FileNotFoundException;
import java.util.LinkedList;
import java.util.Scanner;
import java.util.*;
class BiNode{
public String data;
public BiNode lchild;
public BiNode rchild;
public void BiNode(){
}
public void BiNode(String Data){
this.data=Data;
}
}
class BiTree{
public BiNode root;
public void BiTree(){root=null;}
public BiNode Grow(BiNode node,Scanner scn) {
if(scn.hasNext()){
String temp=scn.next();
if(temp.equals("&")){
scn.close();
return null;
}
else if(temp.equals("#")){
return null;
}
else{
node = new BiNode();
node.data=temp;
System.out.println(node.data);
//System.out.println(this.root.data);
node.lchild=Grow(node.lchild,scn);
node.rchild=Grow(node.rchild,scn);
return node;
}
}
else return null;
}
public void PreOrder(BiNode bt){
if(bt==null) return;
else{
System.out.println(bt.data);
PreOrder(bt.lchild);
PreOrder(bt.rchild);
}
}
public void InOrder(BiNode bt){
if(bt==null) return;
else{
PreOrder(bt.lchild);
System.out.println(bt.data);
PreOrder(bt.rchild);
}
}
public void PostOrder(BiNode bt){
if(bt==null) return;
else{
PreOrder(bt.lchild);
PreOrder(bt.rchild);
System.out.println(bt.data);
}
}
}
public class TestBiTree{
public static void main(String[] args) {
int i=0;
System.out.println("ENTER NOW$$");
Scanner scn=null;
try{
scn= new Scanner(new File("input.txt"));
} catch (FileNotFoundException e) {
e.printStackTrace();
};
BiTree ATree=new BiTree();
ATree.Grow(ATree.root,scn);
System.out.println("IT IS IN NOW$$");
System.out.println(ATree.root.data);
//ATree.PreOrder(ATree.root);
}
}
倒数第三行出错，显示空指针异常，求解答~~

Answer 1

ATree.Grow(ATree.root,scn);
->
ATree.root = ATree.Grow(ATree.root,scn);