Find all possible combinations of k numbers that add up to a numbern, given that only numbers from 1 to 9 can be used and each combination should be a unique set of numbers.

Example 1:

Input: k = 3, n = 7

Output:

[[1,2,4]]
Example 2:
Input: k = 3, n = 9
Output: 
[[1,2,6], [1,3,5], [2,3,4]]
Credits:Special thanks to @mithmatt for adding this PRoblem and creating all test cases.
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class Solution {private:   int index_count;   vector<vector<int> > results;public:    void backtrace(int target, int sum, vector<int>& index, int id, int n, int k)    {        if (sum > target || n > k) {            return;        }        else if (sum == target && n == k) {            vector<int> result;            for(int i = 1; i <= n; ++i)            {                result.push_back(index[i]);            }            results.push_back(result);            return;        }                for (int i = id + 1; i < 10; ++i)        {            index[n+1] = i;            backtrace(target, sum + i, index, i, n+1, k);        }    }    vector<vector<int>> combinationSum3(int k, int n) {        if (k * 9 < n) {            return results;        }                vector<int> index = vector<int>(10, 0);        for (int i = 0; i < 10; ++i) {            index[i] = i;        }        results.clear();        backtrace(n, 0, index, 0, 0, k);                return results;                    }};