216. 组合总和 III

// LeetCode指定调用方法 
public List<List<Integer>> combinationSum3(int k, int n) {
		
    List<List<Integer>> result = new ArrayList<>();

    backtracking(k, n, result, new ArrayList<>(), 1);

    return result;
}

// 回溯，记录当前candidates的开始位置起始元素
private void backtracking(int k, int n, List<List<Integer>> result, List<Integer> combinationSum3List, int t) {

    if (n == 0 || k == 0) {
        if (n == 0 && k == 0) {
            result.add(new ArrayList<>(combinationSum3List));
        }
        return ;
    }

    for (int i = t; i <= 9; i++) {
        if (i > n) {
            break;
        }
        combinationSum3List.add(i);
        backtracking(k - 1, n - i, result, combinationSum3List, i + 1);
        combinationSum3List.remove(combinationSum3List.size() - 1);
    }

}