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;}
};