LeetCode - Combinations

Combinations

public ArrayList<ArrayList<Integer>> combine(int n, int k) {
        ArrayList<ArrayList<Integer>> ret = new ArrayList<ArrayList<Integer>>();
        if(n <= 0 || k <= 0 || n < k) return ret;
        helper(ret, n, k, 1, new ArrayList<Integer>());
        return ret; 
    }
    
    public void helper(ArrayList<ArrayList<Integer>> ret, int n, int k, int start , ArrayList<Integer> lst){
        if(lst.size() == k){
            ret.add(new ArrayList<Integer>(lst));
            return;
        }
        for(int i=start; i <= n; i++){
            lst.add(i);
            helper(ret, n, k, i+1, lst);
            lst.remove(lst.size() - 1);
        }
    }