Solution2.cpp

class Solution {
public:
    vector<vector<int>> combinationSum3(int k, int n) {
        vector<vector<int>> ans;
        vector<int> t;
        function<void(int, int)> dfs = [&](int i, int s) {
            if (s == 0) {
                if (t.size() == k) {
                    ans.emplace_back(t);
                }
                return;
            }
            if (i > 9 || i > s || t.size() >= k) {
                return;
            }
            for (int j = i; j <= 9; ++j) {
                t.emplace_back(j);
                dfs(j + 1, s - j);
                t.pop_back();
            }
        };
        dfs(1, n);
        return ans;
    }
};