[LeetCode] Combinations

Thought:
Basically the same with Combination Sum II, when our cache is full, we add it to result.

Code:
public class Solution {
    public static ArrayList<ArrayList<Integer>> result;
    public static ArrayList<Integer> cache;

    public ArrayList<ArrayList<Integer>> combine(int n, int k) {
        result = new ArrayList<ArrayList<Integer>>();
        cache = new ArrayList<Integer>();
        helper(n, k, 0);
        return result;
    }

    public void helper(int candidates, int target, int index) {
        if(cache.size() == target) {
            result.add(new ArrayList<Integer>(cache));
        }else{
            for(int i = index; i < candidates; i++) {
                cache.add(i + 1);
                helper(candidates, target, i + 1);
                cache.remove(cache.size() - 1);
            }
        }
    }
}