import java.util.List;
import java.util.ArrayList;

public class RecursiveCounter {

    // build a list of the values from 0 to maxNum (exclusive)
    public static List<Integer> buildCounter(int maxNum) {
        List<Integer> nums = new ArrayList<Integer>();
        buildCounterHelper(nums, maxNum, 0);
        return nums;
    }

    // actual recursive helper method
    private static void buildCounterHelper(List<Integer> vals, int maxNum, int curVal) {
        if (curVal == maxNum) {
            return;
        } else {
            vals.add(curVal);
            buildCounterHelper(vals, maxNum, curVal + 1);
        }
    }

    public static void main(String[] args) {
        List<Integer> nums = buildCounter(20);
        System.out.println(nums);
    }

}