// Runtime: 10 ms, faster than 75.67% of Java online submissions for Combinations.
// Memory Usage: 39.3 MB, less than 82.61% of Java online submissions for Combinations.
class Solution {
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> results = new ArrayList<List<Integer>>();
        int[] current = new int[k];
        combine(n, k, 0, current, results);
        return results;
    }

    private void combine(int n, int k, int pos, int[] current, List<List<Integer>> results) {
        if (pos >= k) {
            List<Integer> result = new ArrayList<Integer>();
            for (int x: current) {
                result.add(x);
            }
            results.add(result);
            return;
        }

        int from = (pos == 0) ? 1: current[pos-1] + 1;
        for (int i = from; i <= n; i++) {
            current[pos] = i;
            combine(n, k, pos + 1, current, results);
        }
    }
}