자바 이진트리

import java.util.*;

class Main
{
public static void main(String[] args)
{
int sel;
int num;

Scanner keyboard = new Scanner(System.in);

tree object = new tree();

while(true)
{
System.out.println();
System.out.println(`*********메뉴*********`);
System.out.println();
System.out.println(`1. 추가`);
System.out.println(`2. 출력`);
System.out.println(`3. 전위탐방`);
System.out.println(`4. 중위탐방`);
System.out.println(`5. 후위탐방`);
System.out.println(`6. 종료`);
System.out.println();
System.out.print(`메뉴를 선택해주세요 (1~6) : `);
sel = keyboard.nextInt();
System.out.println();

switch(sel)