法一:利用Integer.toString(n,进制)

import java.util.Scanner;
public class Main {
    
    public static void main(String[] args) {
        
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        System.out.println(Integer.toString(n,6));
    }
}

法二:计算

/*
    120 / 6    20    余0
    20 / 6     3     余2
    3 / 6      0     余3
*/
import java.util.Scanner;
public class Main {
    
    public static void main(String[] args) {
        
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        StringBuilder stringBuilder = new StringBuilder();
        while(n > 0) {
            stringBuilder.append(n%6);
            n /= 6;
        }
        System.out.println(stringBuilder.reverse());
    }
}