class Solution:

    def combine(self, n: int, k: int) -> List[List[int]]:

        res = []

        start = 0

        my_list = []

        self.dfs(n, k, 1, my_list, res)

        return res

    def dfs(self, n, k, start, my_list, res):

        if k == 0:

            res.append(list(my_list))

            return

        for i in range(start, n + 1):

            my_list.append(i)

            # need to use i + 1 instead of start + 1 for the next level

            self.dfs(n, k - 1, i + 1, my_list, res)

            my_list.pop()

class Solution {

    public List<List<Integer>> combine(int n, int k) {

        List<List<Integer>> res = new ArrayList<>();

        helper(res, new ArrayList<Integer>(), n, k, 1);

        return res;

    }

    private void helper(List<List<Integer>> res, List<Integer> list, int n, int k, int start) {

        if (k == 0) {

            res.add(new ArrayList<>(list));

            return;

        }

        // need to go deeper based on current i and ignore the previous result, so that use i + 1

        for (int i = start; i <= n; i++) {

            list.add(i);

            helper(res, list, n, k - 1, i + 1);

            list.remove(list.size() - 1);

        }

    }

}