题目：

题解：

class Solution {
private:
    vector<vector<int>> res;
    void backtracking(int k, int n, vector<int> ans){
        if(k == 0 || n <= 0){
            if(k == 0 && n == 0){
                res.emplace_back(ans);
            }
            return;
        }
        int start = (ans.size() == 0 ? 0 : ans.back()) + 1;
        for(int i = start; i < 10; i++){
            if(n - i < 0)break;  // 加上元素i已经超过n，更大的i也就无须枚举了
            ans.emplace_back(i);
            backtracking(k - 1, n - i, ans);
            ans.pop_back();
        }
    }
public:
    vector<vector<int>> combinationSum3(int k, int n) {
        if(n < (k + 1) * k / 2 || n > (19- k) * k / 2)return res;
        vector<int> ans;
        backtracking(k, n, ans);
        return res;
    }
};