非法访问是什么问题

import java.util.*;



public class Main {


   public static  class Tree12 {

        private Tree12 left;
        private Tree12 right;

        private int data;


        public Tree12(int data) {
            this.data = data;
        }

        public void setLeft(Tree12 left) {
            this.left = left;
        }

        public void setRight(Tree12 right) {
            this.right = right;
        }
        public Tree12 getLeft(){
            return left;
        }
        public Tree12 getRight(){
            return right;
        }
        public int getData(){
            return data;
        }

        public void setData(int data) {
            this.data = data;
        }
        public String toString(){
            return ""+this.data;
        }
        //前序遍历
        public void preOrder(){
            System.out.print(this+" ");
            if(this.left!=null)
                this.left.preOrder();
            if(this.right!=null)
                this.right.preOrder();

        }
        public void prehou(){

            if(this.left!=null)
                this.left.preOrder();

            if(this.right!=null)
                this.right.preOrder();

            System.out.print(this+" ");
        }
        public void prezhong(){
            if(this.left!=null)
                this.left.preOrder();

            System.out.print(this+" ");

            if(this.right!=null)
                this.right.preOrder();

        }

    }


    public static class exam9 {
        private Tree12 root;

        public void createTree(Tree12 t, int fa, int lch, int rch) {
            if(t==null){

                return;
            }else {

                if (t.getData() == fa && t.getData() != 0) {

                    if (lch != 0)
                        t.setLeft(new Tree12(lch));
                    if (rch != 0)
                        t.setRight(new Tree12(rch));
                    //System.out.println("右节点shi"+t.getRight().getData());
                    //System.out.println("节点"+t.getData());
                } else {
                    createTree(t.getLeft(), fa, lch, rch);
                    createTree(t.getRight(), fa, lch, rch);
                }

            }



            //System.out.println("当前父节点"+t.getData());
        }
        public void setRoot(Tree12 root){
            this.root=root;
        }


        public Tree12 getRoot(){
            return root;
        }




    }




    /******************************************************/
    public static void main(String[] args) {
        exam9 e = new exam9();


        Scanner sc = new Scanner(System.in);
        Scanner s = new Scanner(System.in);

        System.out.println("输入节点个数和根节点root");
        int n = s.nextInt();
        int r = s.nextInt();
         Tree12 root=new Tree12(r);
         e.setRoot(root);
        for (int i = 0; i < n ; i++) {
            System.out.println("输入父节点和想插入的左右孩子");
            int fa = sc.nextInt();
            int lch = sc.nextInt();
            int rch = sc.nextInt();
            e.createTree(e.getRoot(), fa, lch, rch);
        }
        //前序遍历
        System.out.println("前序遍历结果为");
        root.preOrder();
        System.out.println();
        //zhong
        System.out.println("中序遍历结果为");
        root.prezhong();
        System.out.println();
        //hou
        System.out.println("后序遍历结果为");
        root.prehou();

    }
}