[LeetCode]Combination Sum III

//DFS，没有剪枝
public class Solution {
    List<List<Integer>> result = new ArrayList<List<Integer>>();
    public List<List<Integer>> combinationSum3(int k, int n) {
        helper(k, n, 0, new ArrayList<Integer>());
        return result;
    }
    public void helper(int k, int n, int left, List<Integer> list) {
        if (k == 1) {
            if (left < n && n <= 9) {
                list.add(n);
                result.add(list);
            }
            return;
        }
        for (int i = left + 1; i <= 9; i++) {
            List<Integer> l = new ArrayList<Integer>(list);
            l.add(i);
            helper(k - 1, n - i, i, l);
        }
    }
}