问题出在哪里了？

public class TenToTwo {
    public static void main(String[] args){
        java.util.Scanner input=new java.util.Scanner(System.in);
        System.out.println("请输入十进制数：");
        String a=input.nextLine();
        String s1=reverse(tentotwo(a));
        System.out.println(a+"的二进制表示为："+s1);

    }
    public static String tentotwo(String a){
        StringBuilder stringBuilder=new StringBuilder();
        int n=Integer.parseInt(a);
        do
        {
        int yu=n%2;
        n=n/2;
        stringBuilder.append(yu);
        }while(n!=0);
        return stringBuilder.toString();
    }
    public static String reverse(String s){
        StringBuilder stringBuilder=new StringBuilder();
        stringBuilder.reverse();
        return stringBuilder.toString();
    }

}