Write function that accepts positive number and prints all numbers with number of digits less than given number. For example input=2 should print 1..9, input=3 should print 1..99 etc.

NumbersTest.java

package com.numberstest;

import org.junit.jupiter.api.Test;

import java.util.Arrays;

public class NumbersTest {
    @Test
    void n() {
        printNumbersBuildOfDigitsLessThan(4);
    }

    private void printNumbersBuildOfDigitsLessThan(int n) {
        for (int i = 1; i < n; i++) {
            printNumbersBuildOfDigits(i);
        }
    }

    private void printNumbersBuildOfDigits(int n) {
        if (n < 1) {
            return;
        }
        int[] num = new int[n];
        num[0] = 1;
        for (int i = 1; i < n; i++) {
            num[i] = 0;
        }

        while (num[0] != 0) {
            System.out.println(join(num));
            int index = n - 1;
            for (; num[index] == 9; index--) {
                num[index] = 0;
                if (index <= 0) {
                    return;
                }
            }
            num[index] = num[index] + 1;
        }
        System.out.println(join(num));
    }

    private static String join(int[] arr) {
        return Arrays.toString(arr).replaceAll("[\\[\\],]", "");
    }

}