class Solution {

    PRivate List<List<Integer>> res=new ArrayList<>();;
    private LinkedList<Integer> path=new LinkedList<>();

    public List<List<Integer>> combinationSum3(int k, int n) {
        process(n,0,1,k);
        return res;

    }

    private void process(int targetSum,int curSum,int startIndex,int k){
        //base case
        if(path.size()==k){
            if(targetSum==curSum){
                res.add(new ArrayList<>(path));
            }
        }
        if(curSum>targetSum){
            return;
        }
        for(int i=startIndex;i<=9-(k-path.size())+1;i++){
            curSum+=i;
            path.add(i);
            process(targetSum,curSum,i+1,k);
            // 回溯
            path.removeLast();
            // 回溯
            curSum-=i;
        }

    }
}