题目链接/文章讲解：https://programmercarl.com/0216.%E7%BB%84%E5%90%88%E6%80%BB%E5%92%8CIII.html

视频讲解：https://www.bilibili.com/video/BV1wg411873x

1.这个题在做完77之后就没什么难度了，除了几个剪枝之外，别的就没什么难度了。

CPP

class Solution {
public:
    vector<int>path;
    vector<vector<int>>res;
    int sum = 0;
    void backtracing(int k, int n, int startIndex){
        if(sum > n){
            return ;
        }
        if(path.size() == k){
            if(sum == n)res.push_back(path);
            return ;
        }
        for(int i = startIndex; i <= 9 - (k - path.size()) + 1; i++){
            path.push_back(i);
            sum += i;
            backtracing(k, n, i+1);
            
            sum -= i;
            path.pop_back();
        }
    }
    vector<vector<int>> combinationSum3(int k, int n) {
        backtracing(k, n, 1);
        return res;
    }
};