Generate subset with k elements

KSubSet.java

public class Solution {
    public ArrayList<ArrayList<Integer>> combine(int n, int k) {
        final ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        if (k == 0) {
            result.add(new ArrayList<Integer>());
            return result;
        }
        for (int i=n; i>=1; --i) {
            for (ArrayList<Integer> ai : combine(i-1, k-1)) {
                ai.add(i);
                result.add(ai);
            }
        }
        return result;
    }
}