77. Combinations

class Solution {
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> res = new ArrayList<>();
        List<Integer> cur = new ArrayList<>();
        int[] number = new int[n];
        for (int i = 0; i < n; i++) {
            number[i] = i + 1;
        }
        helper(n, k, 0, number, cur, res);
        return res;
    }
    
    private void helper(int n, int k, int index, int[] number, List<Integer> cur, List<List<Integer>> res) {
        if (cur.size() > k) {
            return;
        }
        if (cur.size() == k) {
            res.add(new ArrayList<>(cur));
            return;
        }
        for (int i = index; i < n; i++) {
            cur.add(number[i]);
            helper(n, k, i + 1, number, cur, res);
            cur.remove(cur.size() - 1);
        }
    }
}