题目链接

回溯

class Solution {
    List<List<Integer>> res = new ArrayList<List<Integer>>();
    List<Integer> list = new ArrayList<Integer>();

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

    public void backtracking(int k, int n, int sum, int start){
        if(list.size() == k && sum == n){
            res.add(new ArrayList<>(list));
            return ;
        }
        for(int i = start; i <= 9; i++){
            sum += i;
            list.add(i);
            backtracking(k, n, sum, i + 1);
            sum -= i;
            list.removeLast(); 
        }
    }
}