Combination SumIII; BackTracking; Array;

public class Solution {
    public List<List<Integer>> combinationSum3(int k, int n) {
        List<List<Integer>> ret = new ArrayList<>();
        List<Integer> list = new ArrayList<>();
        combination(list, ret, k, n, 0, 1);
        return ret;
    }
    
    public void combination(List<Integer> list, List<List<Integer>> ret, int k, int remain, 
                int last, int index){
        if(remain < last+1) return;
        if(index == k){
            if(remain > 9) return;
            list.add(remain);
            ret.add(new ArrayList(list));
            list.remove(list.size()-1);
            return;
        }
        for(int i = last+1; i <= remain && i <= 9; i++){
                //if(!status) break;
                if(index == 1) list = new ArrayList<>();
                list.add(i);
                combination(list, ret, k, remain-i, i, index+1);
                list.remove(list.size()-1);
        }
    }
    
}