LeetCode Combinations

class Solution {
private:
    vector<vector<int> > res;
public:
    vector<vector<int> > combine(int n, int k) {
        res.clear();
        vector<int> path;
        dfs(1, n, k, path);
        return res;
    }
    
    void dfs(int lower, int upper, int level, vector<int>& path) {
        if (level == 0) {
            res.push_back(path);
            return;
        }
        for (int i=lower; i<= upper - level + 1; i++) {
            path.push_back(i);
            dfs(i + 1, upper, level -1, path);
            path.pop_back();
        }
    }
};