Combinations

public class Solution {
    public ArrayList<ArrayList<Integer>> combine(int n, int k) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        if(n <= 0 || k <= 0 || n<k) return result;
        return helpler(1, n, k);
    }
    private ArrayList<ArrayList<Integer>> helpler(int min, int n, int k)
    {
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        if(k == 0) result.add(new ArrayList<Integer>());
        else{
            for(int i=min; i<=n-k+1; i++)
                for(ArrayList<Integer> temp : helpler(i+1, n, k-1))
                {
                    temp.add(0, i);
                    result.add(temp);
                }
        }
        return result;
    }
}