class Solution {
public:
    vector combine(int n, int k) {
        vector output;
        vector combination;
        combine(output, combination, n, k, 1);
        return output;
    }

private:
    void combine(vector& output, vector combination, int n, int k, int i) {
        if (combination.size() == k) {
            output.push_back(combination);
            return;
        }
        for (int j = i; j